rohrpost

rfc2821.txt (192504B)

---

 
 
 
 
 
 
 Network Working Group                                 J. Klensin, Editor
 Request for Comments: 2821                             AT&T Laboratories
 Obsoletes: 821, 974, 1869                                     April 2001
 Updates: 1123
 Category: Standards Track
 
 
                      Simple Mail Transfer Protocol
 
 Status of this Memo
 
    This document specifies an Internet standards track protocol for the
    Internet community, and requests discussion and suggestions for
    improvements.  Please refer to the current edition of the "Internet
    Official Protocol Standards" (STD 1) for the standardization state
    and status of this protocol.  Distribution of this memo is unlimited.
 
 Copyright Notice
 
    Copyright (C) The Internet Society (2001).  All Rights Reserved.
 
 Abstract
 
    This document is a self-contained specification of the basic protocol
    for the Internet electronic mail transport.  It consolidates, updates
    and clarifies, but doesn't add new or change existing functionality
    of the following:
 
    -  the original SMTP (Simple Mail Transfer Protocol) specification of
       RFC 821 [30],
 
    -  domain name system requirements and implications for mail
       transport from RFC 1035 [22] and RFC 974 [27],
 
    -  the clarifications and applicability statements in RFC 1123 [2],
       and
 
    -  material drawn from the SMTP Extension mechanisms [19].
 
    It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces the
    mail transport materials of RFC 1123).  However, RFC 821 specifies
    some features that were not in significant use in the Internet by the
    mid-1990s and (in appendices) some additional transport models.
    Those sections are omitted here in the interest of clarity and
    brevity; readers needing them should refer to RFC 821.
 
 
 
 
 
 
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    It also includes some additional material from RFC 1123 that required
    amplification.  This material has been identified in multiple ways,
    mostly by tracking flaming on various lists and newsgroups and
    problems of unusual readings or interpretations that have appeared as
    the SMTP extensions have been deployed.  Where this specification
    moves beyond consolidation and actually differs from earlier
    documents, it supersedes them technically as well as textually.
 
    Although SMTP was designed as a mail transport and delivery protocol,
    this specification also contains information that is important to its
    use as a 'mail submission' protocol, as recommended for POP [3, 26]
    and IMAP [6].  Additional submission issues are discussed in RFC 2476
    [15].
 
    Section 2.3 provides definitions of terms specific to this document.
    Except when the historical terminology is necessary for clarity, this
    document uses the current 'client' and 'server' terminology to
    identify the sending and receiving SMTP processes, respectively.
 
    A companion document [32] discusses message headers, message bodies
    and formats and structures for them, and their relationship.
 
 Table of Contents
 
    1. Introduction ..................................................  4
    2. The SMTP Model ................................................  5
    2.1 Basic Structure ..............................................  5
    2.2 The Extension Model ..........................................  7
    2.2.1 Background .................................................  7
    2.2.2 Definition and Registration of Extensions ..................  8
    2.3 Terminology ..................................................  9
    2.3.1 Mail Objects ............................................... 10
    2.3.2 Senders and Receivers ...................................... 10
    2.3.3 Mail Agents and Message Stores ............................. 10
    2.3.4 Host ....................................................... 11
    2.3.5 Domain ..................................................... 11
    2.3.6 Buffer and State Table ..................................... 11
    2.3.7 Lines ...................................................... 12
    2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12
    2.3.9 Message Content and Mail Data .............................. 13
    2.3.10 Mailbox and Address ....................................... 13
    2.3.11 Reply ..................................................... 13
    2.4 General Syntax Principles and Transaction Model .............. 13
    3. The SMTP Procedures: An Overview .............................. 15
    3.1 Session Initiation ........................................... 15
    3.2 Client Initiation ............................................ 16
    3.3 Mail Transactions ............................................ 16
    3.4 Forwarding for Address Correction or Updating ................ 19
 
 
 
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    3.5 Commands for Debugging Addresses ............................. 20
    3.5.1 Overview ................................................... 20
    3.5.2 VRFY Normal Response ....................................... 22
    3.5.3 Meaning of VRFY or EXPN Success Response ................... 22
    3.5.4 Semantics and Applications of EXPN ......................... 23
    3.6 Domains ...................................................... 23
    3.7 Relaying ..................................................... 24
    3.8 Mail Gatewaying .............................................. 25
    3.8.1 Header Fields in Gatewaying ................................ 26
    3.8.2 Received Lines in Gatewaying ............................... 26
    3.8.3 Addresses in Gatewaying .................................... 26
    3.8.4 Other Header Fields in Gatewaying .......................... 27
    3.8.5 Envelopes in Gatewaying .................................... 27
    3.9 Terminating Sessions and Connections ......................... 27
    3.10 Mailing Lists and Aliases ................................... 28
    3.10.1 Alias ..................................................... 28
    3.10.2 List ...................................................... 28
    4. The SMTP Specifications ....................................... 29
    4.1 SMTP Commands ................................................ 29
    4.1.1 Command Semantics and Syntax ............................... 29
    4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO) ................... 29
    4.1.1.2 MAIL (MAIL) .............................................. 31
    4.1.1.3 RECIPIENT (RCPT) ......................................... 31
    4.1.1.4 DATA (DATA) .............................................. 33
    4.1.1.5 RESET (RSET) ............................................. 34
    4.1.1.6 VERIFY (VRFY) ............................................ 35
    4.1.1.7 EXPAND (EXPN) ............................................ 35
    4.1.1.8 HELP (HELP) .............................................. 35
    4.1.1.9 NOOP (NOOP) .............................................. 35
    4.1.1.10 QUIT (QUIT) ............................................. 36
    4.1.2 Command Argument Syntax .................................... 36
    4.1.3 Address Literals ........................................... 38
    4.1.4 Order of Commands .......................................... 39
    4.1.5 Private-use Commands ....................................... 40
    4.2  SMTP Replies ................................................ 40
    4.2.1 Reply Code Severities and Theory ........................... 42
    4.2.2 Reply Codes by Function Groups ............................. 44
    4.2.3  Reply Codes in Numeric Order .............................. 45
    4.2.4 Reply Code 502 ............................................. 46
    4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46
    4.3 Sequencing of Commands and Replies ........................... 47
    4.3.1 Sequencing Overview ........................................ 47
    4.3.2 Command-Reply Sequences .................................... 48
    4.4 Trace Information ............................................ 49
    4.5 Additional Implementation Issues ............................. 53
    4.5.1 Minimum Implementation ..................................... 53
    4.5.2 Transparency ............................................... 53
    4.5.3 Sizes and Timeouts ......................................... 54
 
 
 
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    4.5.3.1 Size limits and minimums ................................. 54
    4.5.3.2 Timeouts ................................................. 56
    4.5.4 Retry Strategies ........................................... 57
    4.5.4.1 Sending Strategy ......................................... 58
    4.5.4.2 Receiving Strategy ....................................... 59
    4.5.5 Messages with a null reverse-path .......................... 59
    5. Address Resolution and Mail Handling .......................... 60
    6. Problem Detection and Handling ................................ 62
    6.1 Reliable Delivery and Replies by Email ....................... 62
    6.2 Loop Detection ............................................... 63
    6.3 Compensating for Irregularities .............................. 63
    7. Security Considerations ....................................... 64
    7.1 Mail Security and Spoofing ................................... 64
    7.2 "Blind" Copies ............................................... 65
    7.3 VRFY, EXPN, and Security ..................................... 65
    7.4 Information Disclosure in Announcements ...................... 66
    7.5 Information Disclosure in Trace Fields ....................... 66
    7.6 Information Disclosure in Message Forwarding ................. 67
    7.7 Scope of Operation of SMTP Servers ........................... 67
    8. IANA Considerations ........................................... 67
    9. References .................................................... 68
    10. Editor's Address ............................................. 70
    11. Acknowledgments .............................................. 70
    Appendices ....................................................... 71
    A. TCP Transport Service ......................................... 71
    B. Generating SMTP Commands from RFC 822 Headers ................. 71
    C. Source Routes ................................................. 72
    D. Scenarios ..................................................... 73
    E. Other Gateway Issues .......................................... 76
    F. Deprecated Features of RFC 821 ................................ 76
    Full Copyright Statement ......................................... 79
 
 1. Introduction
 
    The objective of the Simple Mail Transfer Protocol (SMTP) is to
    transfer mail reliably and efficiently.
 
    SMTP is independent of the particular transmission subsystem and
    requires only a reliable ordered data stream channel.  While this
    document specifically discusses transport over TCP, other transports
    are possible.  Appendices to RFC 821 describe some of them.
 
    An important feature of SMTP is its capability to transport mail
    across networks, usually referred to as "SMTP mail relaying" (see
    section 3.8).  A network consists of the mutually-TCP-accessible
    hosts on the public Internet, the mutually-TCP-accessible hosts on a
    firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
    environment utilizing a non-TCP transport-level protocol.  Using
 
 
 
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    SMTP, a process can transfer mail to another process on the same
    network or to some other network via a relay or gateway process
    accessible to both networks.
 
    In this way, a mail message may pass through a number of intermediate
    relay or gateway hosts on its path from sender to ultimate recipient.
    The Mail eXchanger mechanisms of the domain name system [22, 27] (and
    section 5 of this document) are used to identify the appropriate
    next-hop destination for a message being transported.
 
 2. The SMTP Model
 
 2.1 Basic Structure
 
    The SMTP design can be pictured as:
 
                +----------+                +----------+
    +------+    |          |                |          |
    | User |<-->|          |      SMTP      |          |
    +------+    |  Client- |Commands/Replies| Server-  |
    +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
    | File |<-->|          |    and Mail    |          |<-->| File |
    |System|    |          |                |          |    |System|
    +------+    +----------+                +----------+    +------+
                 SMTP client                SMTP server
 
    When an SMTP client has a message to transmit, it establishes a two-
    way transmission channel to an SMTP server.  The responsibility of an
    SMTP client is to transfer mail messages to one or more SMTP servers,
    or report its failure to do so.
 
    The means by which a mail message is presented to an SMTP client, and
    how that client determines the domain name(s) to which mail messages
    are to be transferred is a local matter, and is not addressed by this
    document.  In some cases, the domain name(s) transferred to, or
    determined by, an SMTP client will identify the final destination(s)
    of the mail message.  In other cases, common with SMTP clients
    associated with implementations of the POP [3, 26] or IMAP [6]
    protocols, or when the SMTP client is inside an isolated transport
    service environment, the domain name determined will identify an
    intermediate destination through which all mail messages are to be
    relayed.  SMTP clients that transfer all traffic, regardless of the
    target domain names associated with the individual messages, or that
    do not maintain queues for retrying message transmissions that
    initially cannot be completed, may otherwise conform to this
    specification but are not considered fully-capable.  Fully-capable
    SMTP implementations, including the relays used by these less capable
 
 
 
 
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    ones, and their destinations, are expected to support all of the
    queuing, retrying, and alternate address functions discussed in this
    specification.
 
    The means by which an SMTP client, once it has determined a target
    domain name, determines the identity of an SMTP server to which a
    copy of a message is to be transferred, and then performs that
    transfer, is covered by this document.  To effect a mail transfer to
    an SMTP server, an SMTP client establishes a two-way transmission
    channel to that SMTP server.  An SMTP client determines the address
    of an appropriate host running an SMTP server by resolving a
    destination domain name to either an intermediate Mail eXchanger host
    or a final target host.
 
    An SMTP server may be either the ultimate destination or an
    intermediate "relay" (that is, it may assume the role of an SMTP
    client after receiving the message) or "gateway" (that is, it may
    transport the message further using some protocol other than SMTP).
    SMTP commands are generated by the SMTP client and sent to the SMTP
    server.  SMTP replies are sent from the SMTP server to the SMTP
    client in response to the commands.
 
    In other words, message transfer can occur in a single connection
    between the original SMTP-sender and the final SMTP-recipient, or can
    occur in a series of hops through intermediary systems.  In either
    case, a formal handoff of responsibility for the message occurs: the
    protocol requires that a server accept responsibility for either
    delivering a message or properly reporting the failure to do so.
 
    Once the transmission channel is established and initial handshaking
    completed, the SMTP client normally initiates a mail transaction.
    Such a transaction consists of a series of commands to specify the
    originator and destination of the mail and transmission of the
    message content (including any headers or other structure) itself.
    When the same message is sent to multiple recipients, this protocol
    encourages the transmission of only one copy of the data for all
    recipients at the same destination (or intermediate relay) host.
 
    The server responds to each command with a reply; replies may
    indicate that the command was accepted, that additional commands are
    expected, or that a temporary or permanent error condition exists.
    Commands specifying the sender or recipients may include server-
    permitted SMTP service extension requests as discussed in section
    2.2.  The dialog is purposely lock-step, one-at-a-time, although this
    can be modified by mutually-agreed extension requests such as command
    pipelining [13].
 
 
 
 
 
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    Once a given mail message has been transmitted, the client may either
    request that the connection be shut down or may initiate other mail
    transactions.  In addition, an SMTP client may use a connection to an
    SMTP server for ancillary services such as verification of email
    addresses or retrieval of mailing list subscriber addresses.
 
    As suggested above, this protocol provides mechanisms for the
    transmission of mail.  This transmission normally occurs directly
    from the sending user's host to the receiving user's host when the
    two hosts are connected to the same transport service.  When they are
    not connected to the same transport service, transmission occurs via
    one or more relay SMTP servers.  An intermediate host that acts as
    either an SMTP relay or as a gateway into some other transmission
    environment is usually selected through the use of the domain name
    service (DNS) Mail eXchanger mechanism.
 
    Usually, intermediate hosts are determined via the DNS MX record, not
    by explicit "source" routing (see section 5 and appendices C and
    F.2).
 
 2.2 The Extension Model
 
 2.2.1 Background
 
    In an effort that started in 1990, approximately a decade after RFC
    821 was completed, the protocol was modified with a "service
    extensions" model that permits the client and server to agree to
    utilize shared functionality beyond the original SMTP requirements.
    The SMTP extension mechanism defines a means whereby an extended SMTP
    client and server may recognize each other, and the server can inform
    the client as to the service extensions that it supports.
 
    Contemporary SMTP implementations MUST support the basic extension
    mechanisms.  For instance, servers MUST support the EHLO command even
    if they do not implement any specific extensions and clients SHOULD
    preferentially utilize EHLO rather than HELO.  (However, for
    compatibility with older conforming implementations, SMTP clients and
    servers MUST support the original HELO mechanisms as a fallback.)
    Unless the different characteristics of HELO must be identified for
    interoperability purposes, this document discusses only EHLO.
 
    SMTP is widely deployed and high-quality implementations have proven
    to be very robust.  However, the Internet community now considers
    some services to be important that were not anticipated when the
    protocol was first designed.  If support for those services is to be
    added, it must be done in a way that permits older implementations to
    continue working acceptably.  The extension framework consists of:
 
 
 
 
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    -  The SMTP command EHLO, superseding the earlier HELO,
 
    -  a registry of SMTP service extensions,
 
    -  additional parameters to the SMTP MAIL and RCPT commands, and
 
    -  optional replacements for commands defined in this protocol, such
       as for DATA in non-ASCII transmissions [33].
 
    SMTP's strength comes primarily from its simplicity.  Experience with
    many protocols has shown that protocols with few options tend towards
    ubiquity, whereas protocols with many options tend towards obscurity.
 
    Each and every extension, regardless of its benefits, must be
    carefully scrutinized with respect to its implementation, deployment,
    and interoperability costs.  In many cases, the cost of extending the
    SMTP service will likely outweigh the benefit.
 
 2.2.2 Definition and Registration of Extensions
 
    The IANA maintains a registry of SMTP service extensions.  A
    corresponding EHLO keyword value is associated with each extension.
    Each service extension registered with the IANA must be defined in a
    formal standards-track or IESG-approved experimental protocol
    document.  The definition must include:
 
    -  the textual name of the SMTP service extension;
 
    -  the EHLO keyword value associated with the extension;
 
    -  the syntax and possible values of parameters associated with the
       EHLO keyword value;
 
    -  any additional SMTP verbs associated with the extension
       (additional verbs will usually be, but are not required to be, the
       same as the EHLO keyword value);
 
    -  any new parameters the extension associates with the MAIL or RCPT
       verbs;
 
    -  a description of how support for the extension affects the
       behavior of a server and client SMTP; and,
 
    -  the increment by which the extension is increasing the maximum
       length of the commands MAIL and/or RCPT, over that specified in
       this standard.
 
 
 
 
 
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    In addition, any EHLO keyword value starting with an upper or lower
    case "X" refers to a local SMTP service extension used exclusively
    through bilateral agreement.  Keywords beginning with "X" MUST NOT be
    used in a registered service extension.  Conversely, keyword values
    presented in the EHLO response that do not begin with "X" MUST
    correspond to a standard, standards-track, or IESG-approved
    experimental SMTP service extension registered with IANA.  A
    conforming server MUST NOT offer non-"X"-prefixed keyword values that
    are not described in a registered extension.
 
    Additional verbs and parameter names are bound by the same rules as
    EHLO keywords; specifically, verbs beginning with "X" are local
    extensions that may not be registered or standardized.  Conversely,
    verbs not beginning with "X" must always be registered.
 
 2.3 Terminology
 
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described below.
 
    1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that
       the definition is an absolute requirement of the specification.
 
    2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the
       definition is an absolute prohibition of the specification.
 
    3. SHOULD   This word, or the adjective "RECOMMENDED", mean that
       there may exist valid reasons in particular circumstances to
       ignore a particular item, but the full implications must be
       understood and carefully weighed before choosing a different
       course.
 
    4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean
       that there may exist valid reasons in particular circumstances
       when the particular behavior is acceptable or even useful, but the
       full implications should be understood and the case carefully
       weighed before implementing any behavior described with this
       label.
 
    5. MAY   This word, or the adjective "OPTIONAL", mean that an item is
       truly optional.  One vendor may choose to include the item because
       a particular marketplace requires it or because the vendor feels
       that it enhances the product while another vendor may omit the
       same item.  An implementation which does not include a particular
       option MUST be prepared to interoperate with another
       implementation which does include the option, though perhaps with
       reduced functionality.  In the same vein an implementation which
 
 
 
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       does include a particular option MUST be prepared to interoperate
       with another implementation which does not include the option
       (except, of course, for the feature the option provides.)
 
 2.3.1 Mail Objects
 
    SMTP transports a mail object.  A mail object contains an envelope
    and content.
 
    The SMTP envelope is sent as a series of SMTP protocol units
    (described in section 3).  It consists of an originator address (to
    which error reports should be directed); one or more recipient
    addresses; and optional protocol extension material.  Historically,
    variations on the recipient address specification command (RCPT TO)
    could be used to specify alternate delivery modes, such as immediate
    display; those variations have now been deprecated (see appendix F,
    section F.6).
 
    The SMTP content is sent in the SMTP DATA protocol unit and has two
    parts:  the headers and the body.  If the content conforms to other
    contemporary standards, the headers form a collection of field/value
    pairs structured as in the message format specification [32]; the
    body, if structured, is defined according to MIME [12].  The content
    is textual in nature, expressed using the US-ASCII repertoire [1].
    Although SMTP extensions (such as "8BITMIME" [20]) may relax this
    restriction for the content body, the content headers are always
    encoded using the US-ASCII repertoire.  A MIME extension [23] defines
    an algorithm for representing header values outside the US-ASCII
    repertoire, while still encoding them using the US-ASCII repertoire.
 
 2.3.2 Senders and Receivers
 
    In RFC 821, the two hosts participating in an SMTP transaction were
    described as the "SMTP-sender" and "SMTP-receiver".  This document
    has been changed to reflect current industry terminology and hence
    refers to them as the "SMTP client" (or sometimes just "the client")
    and "SMTP server" (or just "the server"), respectively.  Since a
    given host may act both as server and client in a relay situation,
    "receiver" and "sender" terminology is still used where needed for
    clarity.
 
 2.3.3 Mail Agents and Message Stores
 
    Additional mail system terminology became common after RFC 821 was
    published and, where convenient, is used in this specification.  In
    particular, SMTP servers and clients provide a mail transport service
    and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User
    Agents" (MUAs or UAs) are normally thought of as the sources and
 
 
 
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    targets of mail.  At the source, an MUA might collect mail to be
    transmitted from a user and hand it off to an MTA; the final
    ("delivery") MTA would be thought of as handing the mail off to an
    MUA (or at least transferring responsibility to it, e.g., by
    depositing the message in a "message store").  However, while these
    terms are used with at least the appearance of great precision in
    other environments, the implied boundaries between MUAs and MTAs
    often do not accurately match common, and conforming, practices with
    Internet mail.  Hence, the reader should be cautious about inferring
    the strong relationships and responsibilities that might be implied
    if these terms were used elsewhere.
 
 2.3.4 Host
 
    For the purposes of this specification, a host is a computer system
    attached to the Internet (or, in some cases, to a private TCP/IP
    network) and supporting the SMTP protocol.  Hosts are known by names
    (see "domain"); identifying them by numerical address is discouraged.
 
 2.3.5 Domain
 
    A domain (or domain name) consists of one or more dot-separated
    components.  These components ("labels" in DNS terminology [22]) are
    restricted for SMTP purposes to consist of a sequence of letters,
    digits, and hyphens drawn from the ASCII character set [1].  Domain
    names are used as names of hosts and of other entities in the domain
    name hierarchy.  For example, a domain may refer to an alias (label
    of a CNAME RR) or the label of Mail eXchanger records to be used to
    deliver mail instead of representing a host name.  See [22] and
    section 5 of this specification.
 
    The domain name, as described in this document and in [22], is the
    entire, fully-qualified name (often referred to as an "FQDN").  A
    domain name that is not in FQDN form is no more than a local alias.
    Local aliases MUST NOT appear in any SMTP transaction.
 
 2.3.6 Buffer and State Table
 
    SMTP sessions are stateful, with both parties carefully maintaining a
    common view of the current state.  In this document we model this
    state by a virtual "buffer" and a "state table" on the server which
    may be used by the client to, for example, "clear the buffer" or
    "reset the state table," causing the information in the buffer to be
    discarded and the state to be returned to some previous state.
 
 
 
 
 
 
 
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 2.3.7 Lines
 
    SMTP commands and, unless altered by a service extension, message
    data, are transmitted in "lines".  Lines consist of zero or more data
    characters terminated by the sequence ASCII character "CR" (hex value
    0D) followed immediately by ASCII character "LF" (hex value 0A).
    This termination sequence is denoted as <CRLF> in this document.
    Conforming implementations MUST NOT recognize or generate any other
    character or character sequence as a line terminator.  Limits MAY be
    imposed on line lengths by servers (see section 4.5.3).
 
    In addition, the appearance of "bare" "CR" or "LF" characters in text
    (i.e., either without the other) has a long history of causing
    problems in mail implementations and applications that use the mail
    system as a tool.  SMTP client implementations MUST NOT transmit
    these characters except when they are intended as line terminators
    and then MUST, as indicated above, transmit them only as a <CRLF>
    sequence.
 
 2.3.8 Originator, Delivery, Relay, and Gateway Systems
 
    This specification makes a distinction among four types of SMTP
    systems, based on the role those systems play in transmitting
    electronic mail.  An "originating" system (sometimes called an SMTP
    originator) introduces mail into the Internet or, more generally,
    into a transport service environment.  A "delivery" SMTP system is
    one that receives mail from a transport service environment and
    passes it to a mail user agent or deposits it in a message store
    which a mail user agent is expected to subsequently access.  A
    "relay" SMTP system (usually referred to just as a "relay") receives
    mail from an SMTP client and transmits it, without modification to
    the message data other than adding trace information, to another SMTP
    server for further relaying or for delivery.
 
    A "gateway" SMTP system (usually referred to just as a "gateway")
    receives mail from a client system in one transport environment and
    transmits it to a server system in another transport environment.
    Differences in protocols or message semantics between the transport
    environments on either side of a gateway may require that the gateway
    system perform transformations to the message that are not permitted
    to SMTP relay systems.  For the purposes of this specification,
    firewalls that rewrite addresses should be considered as gateways,
    even if SMTP is used on both sides of them (see [11]).
 
 
 
 
 
 
 
 
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 2.3.9 Message Content and Mail Data
 
    The terms "message content" and "mail data" are used interchangeably
    in this document to describe the material transmitted after the DATA
    command is accepted and before the end of data indication is
    transmitted.  Message content includes message headers and the
    possibly-structured message body.  The MIME specification [12]
    provides the standard mechanisms for structured message bodies.
 
 2.3.10 Mailbox and Address
 
    As used in this specification, an "address" is a character string
    that identifies a user to whom mail will be sent or a location into
    which mail will be deposited.  The term "mailbox" refers to that
    depository.  The two terms are typically used interchangeably unless
    the distinction between the location in which mail is placed (the
    mailbox) and a reference to it (the address) is important.  An
    address normally consists of user and domain specifications.  The
    standard mailbox naming convention is defined to be "local-
    part@domain": contemporary usage permits a much broader set of
    applications than simple "user names".  Consequently, and due to a
    long history of problems when intermediate hosts have attempted to
    optimize transport by modifying them, the local-part MUST be
    interpreted and assigned semantics only by the host specified in the
    domain part of the address.
 
 2.3.11 Reply
 
    An SMTP reply is an acknowledgment (positive or negative) sent from
    receiver to sender via the transmission channel in response to a
    command.  The general form of a reply is a numeric completion code
    (indicating failure or success) usually followed by a text string.
    The codes are for use by programs and the text is usually intended
    for human users.  Recent work [34] has specified further structuring
    of the reply strings, including the use of supplemental and more
    specific completion codes.
 
 2.4 General Syntax Principles and Transaction Model
 
    SMTP commands and replies have a rigid syntax.  All commands begin
    with a command verb.  All Replies begin with a three digit numeric
    code.  In some commands and replies, arguments MUST follow the verb
    or reply code.  Some commands do not accept arguments (after the
    verb), and some reply codes are followed, sometimes optionally, by
    free form text.  In both cases, where text appears, it is separated
    from the verb or reply code by a space character.  Complete
    definitions of commands and replies appear in section 4.
 
 
 
 
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    Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
    and extension name keywords) are not case sensitive, with the sole
    exception in this specification of a mailbox local-part (SMTP
    Extensions may explicitly specify case-sensitive elements).  That is,
    a command verb, an argument value other than a mailbox local-part,
    and free form text MAY be encoded in upper case, lower case, or any
    mixture of upper and lower case with no impact on its meaning.  This
    is NOT true of a mailbox local-part.  The local-part of a mailbox
    MUST BE treated as case sensitive.  Therefore, SMTP implementations
    MUST take care to preserve the case of mailbox local-parts.  Mailbox
    domains are not case sensitive.  In particular, for some hosts the
    user "smith" is different from the user "Smith".  However, exploiting
    the case sensitivity of mailbox local-parts impedes interoperability
    and is discouraged.
 
    A few SMTP servers, in violation of this specification (and RFC 821)
    require that command verbs be encoded by clients in upper case.
    Implementations MAY wish to employ this encoding to accommodate those
    servers.
 
    The argument field consists of a variable length character string
    ending with the end of the line, i.e., with the character sequence
    <CRLF>.  The receiver will take no action until this sequence is
    received.
 
    The syntax for each command is shown with the discussion of that
    command.  Common elements and parameters are shown in section 4.1.2.
 
    Commands and replies are composed of characters from the ASCII
    character set [1].  When the transport service provides an 8-bit byte
    (octet) transmission channel, each 7-bit character is transmitted
    right justified in an octet with the high order bit cleared to zero.
    More specifically, the unextended SMTP service provides seven bit
    transport only.  An originating SMTP client which has not
    successfully negotiated an appropriate extension with a particular
    server MUST NOT transmit messages with information in the high-order
    bit of octets.  If such messages are transmitted in violation of this
    rule, receiving SMTP servers MAY clear the high-order bit or reject
    the message as invalid.  In general, a relay SMTP SHOULD assume that
    the message content it has received is valid and, assuming that the
    envelope permits doing so, relay it without inspecting that content.
    Of course, if the content is mislabeled and the data path cannot
    accept the actual content, this may result in ultimate delivery of a
    severely garbled message to the recipient.  Delivery SMTP systems MAY
    reject ("bounce") such messages rather than deliver them.  No sending
    SMTP system is permitted to send envelope commands in any character
 
 
 
 
 
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    set other than US-ASCII; receiving systems SHOULD reject such
    commands, normally using "500 syntax error - invalid character"
    replies.
 
    Eight-bit message content transmission MAY be requested of the server
    by a client using extended SMTP facilities, notably the "8BITMIME"
    extension [20].  8BITMIME SHOULD be supported by SMTP servers.
    However, it MUST not be construed as authorization to transmit
    unrestricted eight bit material.  8BITMIME MUST NOT be requested by
    senders for material with the high bit on that is not in MIME format
    with an appropriate content-transfer encoding; servers MAY reject
    such messages.
 
    The metalinguistic notation used in this document corresponds to the
    "Augmented BNF" used in other Internet mail system documents.  The
    reader who is not familiar with that syntax should consult the ABNF
    specification [8].  Metalanguage terms used in running text are
    surrounded by pointed brackets (e.g., <CRLF>) for clarity.
 
 3. The SMTP Procedures: An Overview
 
    This section contains descriptions of the procedures used in SMTP:
    session initiation, the mail transaction, forwarding mail, verifying
    mailbox names and expanding mailing lists, and the opening and
    closing exchanges.  Comments on relaying, a note on mail domains, and
    a discussion of changing roles are included at the end of this
    section.  Several complete scenarios are presented in appendix D.
 
 3.1 Session Initiation
 
    An SMTP session is initiated when a client opens a connection to a
    server and the server responds with an opening message.
 
    SMTP server implementations MAY include identification of their
    software and version information in the connection greeting reply
    after the 220 code, a practice that permits more efficient isolation
    and repair of any problems.  Implementations MAY make provision for
    SMTP servers to disable the software and version announcement where
    it causes security concerns.  While some systems also identify their
    contact point for mail problems, this is not a substitute for
    maintaining the required "postmaster" address (see section 4.5.1).
 
    The SMTP protocol allows a server to formally reject a transaction
    while still allowing the initial connection as follows: a 554
    response MAY be given in the initial connection opening message
    instead of the 220.  A server taking this approach MUST still wait
    for the client to send a QUIT (see section 4.1.1.10) before closing
    the connection and SHOULD respond to any intervening commands with
 
 
 
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    "503 bad sequence of commands".  Since an attempt to make an SMTP
    connection to such a system is probably in error, a server returning
    a 554 response on connection opening SHOULD provide enough
    information in the reply text to facilitate debugging of the sending
    system.
 
 3.2 Client Initiation
 
    Once the server has sent the welcoming message and the client has
    received it, the client normally sends the EHLO command to the
    server, indicating the client's identity.  In addition to opening the
    session, use of EHLO indicates that the client is able to process
    service extensions and requests that the server provide a list of the
    extensions it supports.  Older SMTP systems which are unable to
    support service extensions and contemporary clients which do not
    require service extensions in the mail session being initiated, MAY
    use HELO instead of EHLO.  Servers MUST NOT return the extended
    EHLO-style response to a HELO command.  For a particular connection
    attempt, if the server returns a "command not recognized" response to
    EHLO, the client SHOULD be able to fall back and send HELO.
 
    In the EHLO command the host sending the command identifies itself;
    the command may be interpreted as saying "Hello, I am <domain>" (and,
    in the case of EHLO, "and I support service extension requests").
 
 3.3 Mail Transactions
 
    There are three steps to SMTP mail transactions.  The transaction
    starts with a MAIL command which gives the sender identification.
    (In general, the MAIL command may be sent only when no mail
    transaction is in progress; see section 4.1.4.)  A series of one or
    more RCPT commands follows giving the receiver information.  Then a
    DATA command initiates transfer of the mail data and is terminated by
    the "end of mail" data indicator, which also confirms the
    transaction.
 
    The first step in the procedure is the MAIL command.
 
       MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
 
    This command tells the SMTP-receiver that a new mail transaction is
    starting and to reset all its state tables and buffers, including any
    recipients or mail data.  The <reverse-path> portion of the first or
    only argument contains the source mailbox (between "<" and ">"
    brackets), which can be used to report errors (see section 4.2 for a
    discussion of error reporting).  If accepted, the SMTP server returns
    a 250 OK reply.  If the mailbox specification is not acceptable for
    some reason, the server MUST return a reply indicating whether the
 
 
 
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    failure is permanent (i.e., will occur again if the client tries to
    send the same address again) or temporary (i.e., the address might be
    accepted if the client tries again later).  Despite the apparent
    scope of this requirement, there are circumstances in which the
    acceptability of the reverse-path may not be determined until one or
    more forward-paths (in RCPT commands) can be examined.  In those
    cases, the server MAY reasonably accept the reverse-path (with a 250
    reply) and then report problems after the forward-paths are received
    and examined.  Normally, failures produce 550 or 553 replies.
 
    Historically, the <reverse-path> can contain more than just a
    mailbox, however, contemporary systems SHOULD NOT use source routing
    (see appendix C).
 
    The optional <mail-parameters> are associated with negotiated SMTP
    service extensions (see section 2.2).
 
    The second step in the procedure is the RCPT command.
 
       RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
 
    The first or only argument to this command includes a forward-path
    (normally a mailbox and domain, always surrounded by "<" and ">"
    brackets) identifying one recipient.  If accepted, the SMTP server
    returns a 250 OK reply and stores the forward-path.  If the recipient
    is known not to be a deliverable address, the SMTP server returns a
    550 reply, typically with a string such as "no such user - " and the
    mailbox name (other circumstances and reply codes are possible).
    This step of the procedure can be repeated any number of times.
 
    The <forward-path> can contain more than just a mailbox.
    Historically, the <forward-path> can be a source routing list of
    hosts and the destination mailbox, however, contemporary SMTP clients
    SHOULD NOT utilize source routes (see appendix C).  Servers MUST be
    prepared to encounter a list of source routes in the forward path,
    but SHOULD ignore the routes or MAY decline to support the relaying
    they imply.  Similarly, servers MAY decline to accept mail that is
    destined for other hosts or systems.  These restrictions make a
    server useless as a relay for clients that do not support full SMTP
    functionality.  Consequently, restricted-capability clients MUST NOT
    assume that any SMTP server on the Internet can be used as their mail
    processing (relaying) site.  If a RCPT command appears without a
    previous MAIL command, the server MUST return a 503 "Bad sequence of
    commands" response.  The optional <rcpt-parameters> are associated
    with negotiated SMTP service extensions (see section 2.2).
 
    The third step in the procedure is the DATA command (or some
    alternative specified in a service extension).
 
 
 
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       DATA <CRLF>
 
    If accepted, the SMTP server returns a 354 Intermediate reply and
    considers all succeeding lines up to but not including the end of
    mail data indicator to be the message text.  When the end of text is
    successfully received and stored the SMTP-receiver sends a 250 OK
    reply.
 
    Since the mail data is sent on the transmission channel, the end of
    mail data must be indicated so that the command and reply dialog can
    be resumed.  SMTP indicates the end of the mail data by sending a
    line containing only a "." (period or full stop).  A transparency
    procedure is used to prevent this from interfering with the user's
    text (see section 4.5.2).
 
    The end of mail data indicator also confirms the mail transaction and
    tells the SMTP server to now process the stored recipients and mail
    data.  If accepted, the SMTP server returns a 250 OK reply.  The DATA
    command can fail at only two points in the protocol exchange:
 
    -  If there was no MAIL, or no RCPT, command, or all such commands
       were rejected, the server MAY return a "command out of sequence"
       (503) or "no valid recipients" (554) reply in response to the DATA
       command.  If one of those replies (or any other 5yz reply) is
       received, the client MUST NOT send the message data; more
       generally, message data MUST NOT be sent unless a 354 reply is
       received.
 
    -  If the verb is initially accepted and the 354 reply issued, the
       DATA command should fail only if the mail transaction was
       incomplete (for example, no recipients), or if resources were
       unavailable (including, of course, the server unexpectedly
       becoming unavailable), or if the server determines that the
       message should be rejected for policy or other reasons.
 
    However, in practice, some servers do not perform recipient
    verification until after the message text is received.  These servers
    SHOULD treat a failure for one or more recipients as a "subsequent
    failure" and return a mail message as discussed in section 6.  Using
    a "550 mailbox not found" (or equivalent) reply code after the data
    are accepted makes it difficult or impossible for the client to
    determine which recipients failed.
 
    When RFC 822 format [7, 32] is being used, the mail data include the
    memo header items such as Date, Subject, To, Cc, From.  Server SMTP
    systems SHOULD NOT reject messages based on perceived defects in the
    RFC 822 or MIME [12] message header or message body.  In particular,
 
 
 
 
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    they MUST NOT reject messages in which the numbers of Resent-fields
    do not match or Resent-to appears without Resent-from and/or Resent-
    date.
 
    Mail transaction commands MUST be used in the order discussed above.
 
 3.4 Forwarding for Address Correction or Updating
 
    Forwarding support is most often required to consolidate and simplify
    addresses within, or relative to, some enterprise and less frequently
    to establish addresses to link a person's prior address with current
    one.  Silent forwarding of messages (without server notification to
    the sender), for security or non-disclosure purposes, is common in
    the contemporary Internet.
 
    In both the enterprise and the "new address" cases, information
    hiding (and sometimes security) considerations argue against exposure
    of the "final" address through the SMTP protocol as a side-effect of
    the forwarding activity.  This may be especially important when the
    final address may not even be reachable by the sender.  Consequently,
    the "forwarding" mechanisms described in section 3.2 of RFC 821, and
    especially the 251 (corrected destination) and 551 reply codes from
    RCPT must be evaluated carefully by implementers and, when they are
    available, by those configuring systems.
 
    In particular:
 
    *  Servers MAY forward messages when they are aware of an address
       change.  When they do so, they MAY either provide address-updating
       information with a 251 code, or may forward "silently" and return
       a 250 code.  But, if a 251 code is used, they MUST NOT assume that
       the client will actually update address information or even return
       that information to the user.
 
    Alternately,
 
    *  Servers MAY reject or bounce messages when they are not
       deliverable when addressed.  When they do so, they MAY either
       provide address-updating information with a 551 code, or may
       reject the message as undeliverable with a 550 code and no
       address-specific information.  But, if a 551 code is used, they
       MUST NOT assume that the client will actually update address
       information or even return that information to the user.
 
    SMTP server implementations that support the 251 and/or 551 reply
    codes are strongly encouraged to provide configuration mechanisms so
    that sites which conclude that they would undesirably disclose
    information can disable or restrict their use.
 
 
 
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 3.5 Commands for Debugging Addresses
 
 3.5.1 Overview
 
    SMTP provides commands to verify a user name or obtain the content of
    a mailing list.  This is done with the VRFY and EXPN commands, which
    have character string arguments.  Implementations SHOULD support VRFY
    and EXPN (however, see section 3.5.2 and 7.3).
 
    For the VRFY command, the string is a user name or a user name and
    domain (see below).  If a normal (i.e., 250) response is returned,
    the response MAY include the full name of the user and MUST include
    the mailbox of the user.  It MUST be in either of the following
    forms:
 
       User Name <local-part@domain>
       local-part@domain
 
    When a name that is the argument to VRFY could identify more than one
    mailbox, the server MAY either note the ambiguity or identify the
    alternatives.  In other words, any of the following are legitimate
    response to VRFY:
 
       553 User ambiguous
 
    or
 
       553- Ambiguous;  Possibilities are
       553-Joe Smith <jsmith@foo.com>
       553-Harry Smith <hsmith@foo.com>
       553 Melvin Smith <dweep@foo.com>
 
    or
 
       553-Ambiguous;  Possibilities
       553- <jsmith@foo.com>
       553- <hsmith@foo.com>
       553 <dweep@foo.com>
 
    Under normal circumstances, a client receiving a 553 reply would be
    expected to expose the result to the user.  Use of exactly the forms
    given, and the "user ambiguous" or "ambiguous" keywords, possibly
    supplemented by extended reply codes such as those described in [34],
    will facilitate automated translation into other languages as needed.
    Of course, a client that was highly automated or that was operating
    in another language than English, might choose to try to translate
    the response, to return some other indication to the user than the
 
 
 
 
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    literal text of the reply, or to take some automated action such as
    consulting a directory service for additional information before
    reporting to the user.
 
    For the EXPN command, the string identifies a mailing list, and the
    successful (i.e., 250) multiline response MAY include the full name
    of the users and MUST give the mailboxes on the mailing list.
 
    In some hosts the distinction between a mailing list and an alias for
    a single mailbox is a bit fuzzy, since a common data structure may
    hold both types of entries, and it is possible to have mailing lists
    containing only one mailbox.  If a request is made to apply VRFY to a
    mailing list, a positive response MAY be given if a message so
    addressed would be delivered to everyone on the list, otherwise an
    error SHOULD be reported (e.g., "550 That is a mailing list, not a
    user" or "252 Unable to verify members of mailing list").  If a
    request is made to expand a user name, the server MAY return a
    positive response consisting of a list containing one name, or an
    error MAY be reported (e.g., "550 That is a user name, not a mailing
    list").
 
    In the case of a successful multiline reply (normal for EXPN) exactly
    one mailbox is to be specified on each line of the reply.  The case
    of an ambiguous request is discussed above.
 
    "User name" is a fuzzy term and has been used deliberately.  An
    implementation of the VRFY or EXPN commands MUST include at least
    recognition of local mailboxes as "user names".  However, since
    current Internet practice often results in a single host handling
    mail for multiple domains, hosts, especially hosts that provide this
    functionality, SHOULD accept the "local-part@domain" form as a "user
    name"; hosts MAY also choose to recognize other strings as "user
    names".
 
    The case of expanding a mailbox list requires a multiline reply, such
    as:
 
       C: EXPN Example-People
       S: 250-Jon Postel <Postel@isi.edu>
       S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
       S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
 
    or
 
       C: EXPN Executive-Washroom-List
       S: 550 Access Denied to You.
 
 
 
 
 
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    The character string arguments of the VRFY and EXPN commands cannot
    be further restricted due to the variety of implementations of the
    user name and mailbox list concepts.  On some systems it may be
    appropriate for the argument of the EXPN command to be a file name
    for a file containing a mailing list, but again there are a variety
    of file naming conventions in the Internet.  Similarly, historical
    variations in what is returned by these commands are such that the
    response SHOULD be interpreted very carefully, if at all, and SHOULD
    generally only be used for diagnostic purposes.
 
 3.5.2 VRFY Normal Response
 
    When normal (2yz or 551) responses are returned from a VRFY or EXPN
    request, the reply normally includes the mailbox name, i.e.,
    "<local-part@domain>", where "domain" is a fully qualified domain
    name, MUST appear in the syntax.  In circumstances exceptional enough
    to justify violating the intent of this specification, free-form text
    MAY be returned.  In order to facilitate parsing by both computers
    and people, addresses SHOULD appear in pointed brackets.  When
    addresses, rather than free-form debugging information, are returned,
    EXPN and VRFY MUST return only valid domain addresses that are usable
    in SMTP RCPT commands.  Consequently, if an address implies delivery
    to a program or other system, the mailbox name used to reach that
    target MUST be given.  Paths (explicit source routes) MUST NOT be
    returned by VRFY or EXPN.
 
    Server implementations SHOULD support both VRFY and EXPN.  For
    security reasons, implementations MAY provide local installations a
    way to disable either or both of these commands through configuration
    options or the equivalent.  When these commands are supported, they
    are not required to work across relays when relaying is supported.
    Since they were both optional in RFC 821, they MUST be listed as
    service extensions in an EHLO response, if they are supported.
 
 3.5.3 Meaning of VRFY or EXPN Success Response
 
    A server MUST NOT return a 250 code in response to a VRFY or EXPN
    command unless it has actually verified the address.  In particular,
    a server MUST NOT return 250 if all it has done is to verify that the
    syntax given is valid.  In that case, 502 (Command not implemented)
    or 500 (Syntax error, command unrecognized) SHOULD be returned.  As
    stated elsewhere, implementation (in the sense of actually validating
    addresses and returning information) of VRFY and EXPN are strongly
    recommended.  Hence, implementations that return 500 or 502 for VRFY
    are not in full compliance with this specification.
 
 
 
 
 
 
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    There may be circumstances where an address appears to be valid but
    cannot reasonably be verified in real time, particularly when a
    server is acting as a mail exchanger for another server or domain.
    "Apparent validity" in this case would normally involve at least
    syntax checking and might involve verification that any domains
    specified were ones to which the host expected to be able to relay
    mail.  In these situations, reply code 252 SHOULD be returned.  These
    cases parallel the discussion of RCPT verification discussed in
    section 2.1.  Similarly, the discussion in section 3.4 applies to the
    use of reply codes 251 and 551 with VRFY (and EXPN) to indicate
    addresses that are recognized but that would be forwarded or bounced
    were mail received for them.  Implementations generally SHOULD be
    more aggressive about address verification in the case of VRFY than
    in the case of RCPT, even if it takes a little longer to do so.
 
 3.5.4 Semantics and Applications of EXPN
 
    EXPN is often very useful in debugging and understanding problems
    with mailing lists and multiple-target-address aliases.  Some systems
    have attempted to use source expansion of mailing lists as a means of
    eliminating duplicates.  The propagation of aliasing systems with
    mail on the Internet, for hosts (typically with MX and CNAME DNS
    records), for mailboxes (various types of local host aliases), and in
    various proxying arrangements, has made it nearly impossible for
    these strategies to work consistently, and mail systems SHOULD NOT
    attempt them.
 
 3.6 Domains
 
    Only resolvable, fully-qualified, domain names (FQDNs) are permitted
    when domain names are used in SMTP.  In other words, names that can
    be resolved to MX RRs or A RRs (as discussed in section 5) are
    permitted, as are CNAME RRs whose targets can be resolved, in turn,
    to MX or A RRs.  Local nicknames or unqualified names MUST NOT be
    used.  There are two exceptions to the rule requiring FQDNs:
 
    -  The domain name given in the EHLO command MUST BE either a primary
       host name (a domain name that resolves to an A RR) or, if the host
       has no name, an address literal as described in section 4.1.1.1.
 
    -  The reserved mailbox name "postmaster" may be used in a RCPT
       command without domain qualification (see section 4.1.1.3) and
       MUST be accepted if so used.
 
 
 
 
 
 
 
 
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 3.7 Relaying
 
    In general, the availability of Mail eXchanger records in the domain
    name system [22, 27] makes the use of explicit source routes in the
    Internet mail system unnecessary.  Many historical problems with
    their interpretation have made their use undesirable.  SMTP clients
    SHOULD NOT generate explicit source routes except under unusual
    circumstances.  SMTP servers MAY decline to act as mail relays or to
    accept addresses that specify source routes.  When route information
    is encountered, SMTP servers are also permitted to ignore the route
    information and simply send to the final destination specified as the
    last element in the route and SHOULD do so.  There has been an
    invalid practice of using names that do not appear in the DNS as
    destination names, with the senders counting on the intermediate
    hosts specified in source routing to resolve any problems.  If source
    routes are stripped, this practice will cause failures.  This is one
    of several reasons why SMTP clients MUST NOT generate invalid source
    routes or depend on serial resolution of names.
 
    When source routes are not used, the process described in RFC 821 for
    constructing a reverse-path from the forward-path is not applicable
    and the reverse-path at the time of delivery will simply be the
    address that appeared in the MAIL command.
 
    A relay SMTP server is usually the target of a DNS MX record that
    designates it, rather than the final delivery system.  The relay
    server may accept or reject the task of relaying the mail in the same
    way it accepts or rejects mail for a local user.  If it accepts the
    task, it then becomes an SMTP client, establishes a transmission
    channel to the next SMTP server specified in the DNS (according to
    the rules in section 5), and sends it the mail.  If it declines to
    relay mail to a particular address for policy reasons, a 550 response
    SHOULD be returned.
 
    Many mail-sending clients exist, especially in conjunction with
    facilities that receive mail via POP3 or IMAP, that have limited
    capability to support some of the requirements of this specification,
    such as the ability to queue messages for subsequent delivery
    attempts.  For these clients, it is common practice to make private
    arrangements to send all messages to a single server for processing
    and subsequent distribution.  SMTP, as specified here, is not ideally
    suited for this role, and work is underway on standardized mail
    submission protocols that might eventually supercede the current
    practices.  In any event, because these arrangements are private and
    fall outside the scope of this specification, they are not described
    here.
 
 
 
 
 
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    It is important to note that MX records can point to SMTP servers
    which act as gateways into other environments, not just SMTP relays
    and final delivery systems; see sections 3.8 and 5.
 
    If an SMTP server has accepted the task of relaying the mail and
    later finds that the destination is incorrect or that the mail cannot
    be delivered for some other reason, then it MUST construct an
    "undeliverable mail" notification message and send it to the
    originator of the undeliverable mail (as indicated by the reverse-
    path).  Formats specified for non-delivery reports by other standards
    (see, for example, [24, 25]) SHOULD be used if possible.
 
    This notification message must be from the SMTP server at the relay
    host or the host that first determines that delivery cannot be
    accomplished.  Of course, SMTP servers MUST NOT send notification
    messages about problems transporting notification messages.  One way
    to prevent loops in error reporting is to specify a null reverse-path
    in the MAIL command of a notification message.  When such a message
    is transmitted the reverse-path MUST be set to null (see section
    4.5.5 for additional discussion).  A MAIL command with a null
    reverse-path appears as follows:
 
       MAIL FROM:<>
 
    As discussed in section 2.4.1, a relay SMTP has no need to inspect or
    act upon the headers or body of the message data and MUST NOT do so
    except to add its own "Received:" header (section 4.4) and,
    optionally, to attempt to detect looping in the mail system (see
    section 6.2).
 
 3.8 Mail Gatewaying
 
    While the relay function discussed above operates within the Internet
    SMTP transport service environment, MX records or various forms of
    explicit routing may require that an intermediate SMTP server perform
    a translation function between one transport service and another.  As
    discussed in section 2.3.8, when such a system is at the boundary
    between two transport service environments, we refer to it as a
    "gateway" or "gateway SMTP".
 
    Gatewaying mail between different mail environments, such as
    different mail formats and protocols, is complex and does not easily
    yield to standardization.  However, some general requirements may be
    given for a gateway between the Internet and another mail
    environment.
 
 
 
 
 
 
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 3.8.1 Header Fields in Gatewaying
 
    Header fields MAY be rewritten when necessary as messages are
    gatewayed across mail environment boundaries.  This may involve
    inspecting the message body or interpreting the local-part of the
    destination address in spite of the prohibitions in section 2.4.1.
 
    Other mail systems gatewayed to the Internet often use a subset of
    RFC 822 headers or provide similar functionality with a different
    syntax, but some of these mail systems do not have an equivalent to
    the SMTP envelope.  Therefore, when a message leaves the Internet
    environment, it may be necessary to fold the SMTP envelope
    information into the message header.  A possible solution would be to
    create new header fields to carry the envelope information (e.g.,
    "X-SMTP-MAIL:"  and "X-SMTP-RCPT:"); however, this would require
    changes in mail programs in foreign environments and might risk
    disclosure of private information (see section 7.2).
 
 3.8.2 Received Lines in Gatewaying
 
    When forwarding a message into or out of the Internet environment, a
    gateway MUST prepend a Received: line, but it MUST NOT alter in any
    way a Received: line that is already in the header.
 
    "Received:" fields of messages originating from other environments
    may not conform exactly to this specification.  However, the most
    important use of Received: lines is for debugging mail faults, and
    this debugging can be severely hampered by well-meaning gateways that
    try to "fix" a Received: line.  As another consequence of trace
    fields arising in non-SMTP environments, receiving systems MUST NOT
    reject mail based on the format of a trace field and SHOULD be
    extremely robust in the light of unexpected information or formats in
    those fields.
 
    The gateway SHOULD indicate the environment and protocol in the "via"
    clauses of Received field(s) that it supplies.
 
 3.8.3 Addresses in Gatewaying
 
    From the Internet side, the gateway SHOULD accept all valid address
    formats in SMTP commands and in RFC 822 headers, and all valid RFC
    822 messages.  Addresses and headers generated by gateways MUST
    conform to applicable Internet standards (including this one and RFC
    822).  Gateways are, of course, subject to the same rules for
    handling source routes as those described for other SMTP systems in
    section 3.3.
 
 
 
 
 
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 3.8.4 Other Header Fields in Gatewaying
 
    The gateway MUST ensure that all header fields of a message that it
    forwards into the Internet mail environment meet the requirements for
    Internet mail.  In particular, all addresses in "From:", "To:",
    "Cc:", etc., fields MUST be transformed (if necessary) to satisfy RFC
    822 syntax, MUST reference only fully-qualified domain names, and
    MUST be effective and useful for sending replies.  The translation
    algorithm used to convert mail from the Internet protocols to another
    environment's protocol SHOULD ensure that error messages from the
    foreign mail environment are delivered to the return path from the
    SMTP envelope, not to the sender listed in the "From:" field (or
    other fields) of the RFC 822 message.
 
 3.8.5 Envelopes in Gatewaying
 
    Similarly, when forwarding a message from another environment into
    the Internet, the gateway SHOULD set the envelope return path in
    accordance with an error message return address, if supplied by the
    foreign environment.  If the foreign environment has no equivalent
    concept, the gateway must select and use a best approximation, with
    the message originator's address as the default of last resort.
 
 3.9 Terminating Sessions and Connections
 
    An SMTP connection is terminated when the client sends a QUIT
    command.  The server responds with a positive reply code, after which
    it closes the connection.
 
    An SMTP server MUST NOT intentionally close the connection except:
 
    -  After receiving a QUIT command and responding with a 221 reply.
 
    -  After detecting the need to shut down the SMTP service and
       returning a 421 response code.  This response code can be issued
       after the server receives any command or, if necessary,
       asynchronously from command receipt (on the assumption that the
       client will receive it after the next command is issued).
 
    In particular, a server that closes connections in response to
    commands that are not understood is in violation of this
    specification.  Servers are expected to be tolerant of unknown
    commands, issuing a 500 reply and awaiting further instructions from
    the client.
 
 
 
 
 
 
 
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    An SMTP server which is forcibly shut down via external means SHOULD
    attempt to send a line containing a 421 response code to the SMTP
    client before exiting.  The SMTP client will normally read the 421
    response code after sending its next command.
 
    SMTP clients that experience a connection close, reset, or other
    communications failure due to circumstances not under their control
    (in violation of the intent of this specification but sometimes
    unavoidable) SHOULD, to maintain the robustness of the mail system,
    treat the mail transaction as if a 451 response had been received and
    act accordingly.
 
 3.10 Mailing Lists and Aliases
 
    An SMTP-capable host SHOULD support both the alias and the list
    models of address expansion for multiple delivery.  When a message is
    delivered or forwarded to each address of an expanded list form, the
    return address in the envelope ("MAIL FROM:") MUST be changed to be
    the address of a person or other entity who administers the list.
    However, in this case, the message header [32] MUST be left
    unchanged; in particular, the "From" field of the message header is
    unaffected.
 
    An important mail facility is a mechanism for multi-destination
    delivery of a single message, by transforming (or "expanding" or
    "exploding") a pseudo-mailbox address into a list of destination
    mailbox addresses.  When a message is sent to such a pseudo-mailbox
    (sometimes called an "exploder"), copies are forwarded or
    redistributed to each mailbox in the expanded list.  Servers SHOULD
    simply utilize the addresses on the list; application of heuristics
    or other matching rules to eliminate some addresses, such as that of
    the originator, is strongly discouraged.  We classify such a pseudo-
    mailbox as an "alias" or a "list", depending upon the expansion
    rules.
 
 3.10.1 Alias
 
    To expand an alias, the recipient mailer simply replaces the pseudo-
    mailbox address in the envelope with each of the expanded addresses
    in turn; the rest of the envelope and the message body are left
    unchanged.  The message is then delivered or forwarded to each
    expanded address.
 
 3.10.2 List
 
    A mailing list may be said to operate by "redistribution" rather than
    by "forwarding".  To expand a list, the recipient mailer replaces the
    pseudo-mailbox address in the envelope with all of the expanded
 
 
 
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    addresses.  The return address in the envelope is changed so that all
    error messages generated by the final deliveries will be returned to
    a list administrator, not to the message originator, who generally
    has no control over the contents of the list and will typically find
    error messages annoying.
 
 4. The SMTP Specifications
 
 4.1 SMTP Commands
 
 4.1.1 Command Semantics and Syntax
 
    The SMTP commands define the mail transfer or the mail system
    function requested by the user.  SMTP commands are character strings
    terminated by <CRLF>.  The commands themselves are alphabetic
    characters terminated by <SP> if parameters follow and <CRLF>
    otherwise.  (In the interest of improved interoperability, SMTP
    receivers are encouraged to tolerate trailing white space before the
    terminating <CRLF>.)  The syntax of the local part of a mailbox must
    conform to receiver site conventions and the syntax specified in
    section 4.1.2.  The SMTP commands are discussed below.  The SMTP
    replies are discussed in section 4.2.
 
    A mail transaction involves several data objects which are
    communicated as arguments to different commands.  The reverse-path is
    the argument of the MAIL command, the forward-path is the argument of
    the RCPT command, and the mail data is the argument of the DATA
    command.  These arguments or data objects must be transmitted and
    held pending the confirmation communicated by the end of mail data
    indication which finalizes the transaction.  The model for this is
    that distinct buffers are provided to hold the types of data objects,
    that is, there is a reverse-path buffer, a forward-path buffer, and a
    mail data buffer.  Specific commands cause information to be appended
    to a specific buffer, or cause one or more buffers to be cleared.
 
    Several commands (RSET, DATA, QUIT) are specified as not permitting
    parameters.  In the absence of specific extensions offered by the
    server and accepted by the client, clients MUST NOT send such
    parameters and servers SHOULD reject commands containing them as
    having invalid syntax.
 
 4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO)
 
    These commands are used to identify the SMTP client to the SMTP
    server.  The argument field contains the fully-qualified domain name
    of the SMTP client if one is available.  In situations in which the
    SMTP client system does not have a meaningful domain name (e.g., when
    its address is dynamically allocated and no reverse mapping record is
 
 
 
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    available), the client SHOULD send an address literal (see section
    4.1.3), optionally followed by information that will help to identify
    the client system.  y The SMTP server identifies itself to the SMTP
    client in the connection greeting reply and in the response to this
    command.
 
    A client SMTP SHOULD start an SMTP session by issuing the EHLO
    command.  If the SMTP server supports the SMTP service extensions it
    will give a successful response, a failure response, or an error
    response.  If the SMTP server, in violation of this specification,
    does not support any SMTP service extensions it will generate an
    error response.  Older client SMTP systems MAY, as discussed above,
    use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
    support the HELO command and reply properly to it.  In any event, a
    client MUST issue HELO or EHLO before starting a mail transaction.
 
    These commands, and a "250 OK" reply to one of them, confirm that
    both the SMTP client and the SMTP server are in the initial state,
    that is, there is no transaction in progress and all state tables and
    buffers are cleared.
 
    Syntax:
 
       ehlo            = "EHLO" SP Domain CRLF
       helo            = "HELO" SP Domain CRLF
 
    Normally, the response to EHLO will be a multiline reply.  Each line
    of the response contains a keyword and, optionally, one or more
    parameters.  Following the normal syntax for multiline replies, these
    keyworks follow the code (250) and a hyphen for all but the last
    line, and the code and a space for the last line.  The syntax for a
    positive response, using the ABNF notation and terminal symbols of
    [8], is:
 
       ehlo-ok-rsp  =    ( "250"    domain [ SP ehlo-greet ] CRLF )
                    / (    "250-"   domain [ SP ehlo-greet ] CRLF
                        *( "250-"   ehlo-line                CRLF )
                           "250"    SP ehlo-line             CRLF  )
 
       ehlo-greet   = 1*(%d0-9 / %d11-12 / %d14-127)
                    ; string of any characters other than CR or LF
 
       ehlo-line    = ehlo-keyword *( SP ehlo-param )
 
       ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                    ; additional syntax of ehlo-params depends on
                    ; ehlo-keyword
 
 
 
 
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       ehlo-param   = 1*(%d33-127)
                    ; any CHAR excluding <SP> and all
                    ; control characters (US-ASCII 0-31 inclusive)
 
    Although EHLO keywords may be specified in upper, lower, or mixed
    case, they MUST always be recognized and processed in a case-
    insensitive manner.  This is simply an extension of practices
    specified in RFC 821 and section 2.4.1.
 
 4.1.1.2 MAIL (MAIL)
 
    This command is used to initiate a mail transaction in which the mail
    data is delivered to an SMTP server which may, in turn, deliver it to
    one or more mailboxes or pass it on to another system (possibly using
    SMTP).  The argument field contains a reverse-path and may contain
    optional parameters.  In general, the MAIL command may be sent only
    when no mail transaction is in progress, see section 4.1.4.
 
    The reverse-path consists of the sender mailbox.  Historically, that
    mailbox might optionally have been preceded by a list of hosts, but
    that behavior is now deprecated (see appendix C).  In some types of
    reporting messages for which a reply is likely to cause a mail loop
    (for example, mail delivery and nondelivery notifications), the
    reverse-path may be null (see section 3.7).
 
    This command clears the reverse-path buffer, the forward-path buffer,
    and the mail data buffer; and inserts the reverse-path information
    from this command into the reverse-path buffer.
 
    If service extensions were negotiated, the MAIL command may also
    carry parameters associated with a particular service extension.
 
    Syntax:
 
       "MAIL FROM:" ("<>" / Reverse-Path)
                        [SP Mail-parameters] CRLF
 
 4.1.1.3 RECIPIENT (RCPT)
 
    This command is used to identify an individual recipient of the mail
    data; multiple recipients are specified by multiple use of this
    command.  The argument field contains a forward-path and may contain
    optional parameters.
 
    The forward-path normally consists of the required destination
    mailbox.  Sending systems SHOULD not generate the optional list of
    hosts known as a source route.  Receiving systems MUST recognize
 
 
 
 
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    source route syntax but SHOULD strip off the source route
    specification and utilize the domain name associated with the mailbox
    as if the source route had not been provided.
 
    Similarly, relay hosts SHOULD strip or ignore source routes, and
    names MUST NOT be copied into the reverse-path.  When mail reaches
    its ultimate destination (the forward-path contains only a
    destination mailbox), the SMTP server inserts it into the destination
    mailbox in accordance with its host mail conventions.
 
    For example, mail received at relay host xyz.com with envelope
    commands
 
       MAIL FROM:<userx@y.foo.org>
       RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
 
    will normally be sent directly on to host d.bar.org with envelope
    commands
 
       MAIL FROM:<userx@y.foo.org>
       RCPT TO:<userc@d.bar.org>
 
    As provided in appendix C, xyz.com MAY also choose to relay the
    message to hosta.int, using the envelope commands
 
       MAIL FROM:<userx@y.foo.org>
       RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
 
    or to jkl.org, using the envelope commands
 
       MAIL FROM:<userx@y.foo.org>
       RCPT TO:<@jkl.org:userc@d.bar.org>
 
    Of course, since hosts are not required to relay mail at all, xyz.com
    may also reject the message entirely when the RCPT command is
    received, using a 550 code (since this is a "policy reason").
 
    If service extensions were negotiated, the RCPT command may also
    carry parameters associated with a particular service extension
    offered by the server.  The client MUST NOT transmit parameters other
    than those associated with a service extension offered by the server
    in its EHLO response.
 
 Syntax:
    "RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)
                     [SP Rcpt-parameters] CRLF
 
 
 
 
 
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 4.1.1.4 DATA (DATA)
 
    The receiver normally sends a 354 response to DATA, and then treats
    the lines (strings ending in <CRLF> sequences, as described in
    section 2.3.7) following the command as mail data from the sender.
    This command causes the mail data to be appended to the mail data
    buffer.  The mail data may contain any of the 128 ASCII character
    codes, although experience has indicated that use of control
    characters other than SP, HT, CR, and LF may cause problems and
    SHOULD be avoided when possible.
 
    The mail data is terminated by a line containing only a period, that
    is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2).  This
    is the end of mail data indication.  Note that the first <CRLF> of
    this terminating sequence is also the <CRLF> that ends the final line
    of the data (message text) or, if there was no data, ends the DATA
    command itself.  An extra <CRLF> MUST NOT be added, as that would
    cause an empty line to be added to the message.  The only exception
    to this rule would arise if the message body were passed to the
    originating SMTP-sender with a final "line" that did not end in
    <CRLF>; in that case, the originating SMTP system MUST either reject
    the message as invalid or add <CRLF> in order to have the receiving
    SMTP server recognize the "end of data" condition.
 
    The custom of accepting lines ending only in <LF>, as a concession to
    non-conforming behavior on the part of some UNIX systems, has proven
    to cause more interoperability problems than it solves, and SMTP
    server systems MUST NOT do this, even in the name of improved
    robustness.  In particular, the sequence "<LF>.<LF>" (bare line
    feeds, without carriage returns) MUST NOT be treated as equivalent to
    <CRLF>.<CRLF> as the end of mail data indication.
 
    Receipt of the end of mail data indication requires the server to
    process the stored mail transaction information.  This processing
    consumes the information in the reverse-path buffer, the forward-path
    buffer, and the mail data buffer, and on the completion of this
    command these buffers are cleared.  If the processing is successful,
    the receiver MUST send an OK reply.  If the processing fails the
    receiver MUST send a failure reply.  The SMTP model does not allow
    for partial failures at this point: either the message is accepted by
    the server for delivery and a positive response is returned or it is
    not accepted and a failure reply is returned.  In sending a positive
    completion reply to the end of data indication, the receiver takes
    full responsibility for the message (see section 6.1).  Errors that
    are diagnosed subsequently MUST be reported in a mail message, as
    discussed in section 4.4.
 
 
 
 
 
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    When the SMTP server accepts a message either for relaying or for
    final delivery, it inserts a trace record (also referred to
    interchangeably as a "time stamp line" or "Received" line) at the top
    of the mail data.  This trace record indicates the identity of the
    host that sent the message, the identity of the host that received
    the message (and is inserting this time stamp), and the date and time
    the message was received.  Relayed messages will have multiple time
    stamp lines.  Details for formation of these lines, including their
    syntax, is specified in section 4.4.
 
    Additional discussion about the operation of the DATA command appears
    in section 3.3.
 
    Syntax:
       "DATA" CRLF
 
 4.1.1.5 RESET (RSET)
 
    This command specifies that the current mail transaction will be
    aborted.  Any stored sender, recipients, and mail data MUST be
    discarded, and all buffers and state tables cleared.  The receiver
    MUST send a "250 OK" reply to a RSET command with no arguments.  A
    reset command may be issued by the client at any time.  It is
    effectively equivalent to a NOOP (i.e., if has no effect) if issued
    immediately after EHLO, before EHLO is issued in the session, after
    an end-of-data indicator has been sent and acknowledged, or
    immediately before a QUIT.  An SMTP server MUST NOT close the
    connection as the result of receiving a RSET; that action is reserved
    for QUIT (see section 4.1.1.10).
 
    Since EHLO implies some additional processing and response by the
    server, RSET will normally be more efficient than reissuing that
    command, even though the formal semantics are the same.
 
    There are circumstances, contrary to the intent of this
    specification, in which an SMTP server may receive an indication that
    the underlying TCP connection has been closed or reset.  To preserve
    the robustness of the mail system, SMTP servers SHOULD be prepared
    for this condition and SHOULD treat it as if a QUIT had been received
    before the connection disappeared.
 
    Syntax:
       "RSET" CRLF
 
 
 
 
 
 
 
 
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 4.1.1.6 VERIFY (VRFY)
 
    This command asks the receiver to confirm that the argument
    identifies a user or mailbox.  If it is a user name, information is
    returned as specified in section 3.5.
 
    This command has no effect on the reverse-path buffer, the forward-
    path buffer, or the mail data buffer.
 
    Syntax:
       "VRFY" SP String CRLF
 
 4.1.1.7 EXPAND (EXPN)
 
    This command asks the receiver to confirm that the argument
    identifies a mailing list, and if so, to return the membership of
    that list.  If the command is successful, a reply is returned
    containing information as described in section 3.5.  This reply will
    have multiple lines except in the trivial case of a one-member list.
 
    This command has no effect on the reverse-path buffer, the forward-
    path buffer, or the mail data buffer and may be issued at any time.
 
    Syntax:
       "EXPN" SP String CRLF
 
 4.1.1.8 HELP (HELP)
 
    This command causes the server to send helpful information to the
    client.  The command MAY take an argument (e.g., any command name)
    and return more specific information as a response.
 
    This command has no effect on the reverse-path buffer, the forward-
    path buffer, or the mail data buffer and may be issued at any time.
 
    SMTP servers SHOULD support HELP without arguments and MAY support it
    with arguments.
 
    Syntax:
       "HELP" [ SP String ] CRLF
 
 4.1.1.9 NOOP (NOOP)
 
    This command does not affect any parameters or previously entered
    commands.  It specifies no action other than that the receiver send
    an OK reply.
 
 
 
 
 
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    This command has no effect on the reverse-path buffer, the forward-
    path buffer, or the mail data buffer and may be issued at any time.
    If a parameter string is specified, servers SHOULD ignore it.
 
    Syntax:
       "NOOP" [ SP String ] CRLF
 
 4.1.1.10 QUIT (QUIT)
 
    This command specifies that the receiver MUST send an OK reply, and
    then close the transmission channel.
 
    The receiver MUST NOT intentionally close the transmission channel
    until it receives and replies to a QUIT command (even if there was an
    error).  The sender MUST NOT intentionally close the transmission
    channel until it sends a QUIT command and SHOULD wait until it
    receives the reply (even if there was an error response to a previous
    command).  If the connection is closed prematurely due to violations
    of the above or system or network failure, the server MUST cancel any
    pending transaction, but not undo any previously completed
    transaction, and generally MUST act as if the command or transaction
    in progress had received a temporary error (i.e., a 4yz response).
 
    The QUIT command may be issued at any time.
 
    Syntax:
       "QUIT" CRLF
 
 4.1.2 Command Argument Syntax
 
    The syntax of the argument fields of the above commands (using the
    syntax specified in [8] where applicable) is given below.  Some of
    the productions given below are used only in conjunction with source
    routes as described in appendix C.  Terminals not defined in this
    document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in
    the "core" syntax [8 (section 6)] or in the message format syntax
    [32].
 
       Reverse-path = Path
       Forward-path = Path
       Path = "<" [ A-d-l ":" ] Mailbox ">"
       A-d-l = At-domain *( "," A-d-l )
             ; Note that this form, the so-called "source route",
             ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
             ; ignored.
       At-domain = "@" domain
       Mail-parameters = esmtp-param *(SP esmtp-param)
       Rcpt-parameters = esmtp-param *(SP esmtp-param)
 
 
 
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       esmtp-param     = esmtp-keyword ["=" esmtp-value]
       esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
       esmtp-value     = 1*(%d33-60 / %d62-127)
             ; any CHAR excluding "=", SP, and control characters
       Keyword  = Ldh-str
       Argument = Atom
       Domain = (sub-domain 1*("." sub-domain)) / address-literal
       sub-domain = Let-dig [Ldh-str]
 
       address-literal = "[" IPv4-address-literal /
                             IPv6-address-literal /
                             General-address-literal "]"
             ; See section 4.1.3
 
       Mailbox = Local-part "@" Domain
 
       Local-part = Dot-string / Quoted-string
             ; MAY be case-sensitive
 
       Dot-string = Atom *("." Atom)
 
       Atom = 1*atext
 
       Quoted-string = DQUOTE *qcontent DQUOTE
 
       String = Atom / Quoted-string
 
    While the above definition for Local-part is relatively permissive,
    for maximum interoperability, a host that expects to receive mail
    SHOULD avoid defining mailboxes where the Local-part requires (or
    uses) the Quoted-string form or where the Local-part is case-
    sensitive.  For any purposes that require generating or comparing
    Local-parts (e.g., to specific mailbox names), all quoted forms MUST
    be treated as equivalent and the sending system SHOULD transmit the
    form that uses the minimum quoting possible.
 
    Systems MUST NOT define mailboxes in such a way as to require the use
    in SMTP of non-ASCII characters (octets with the high order bit set
    to one) or ASCII "control characters" (decimal value 0-31 and 127).
    These characters MUST NOT be used in MAIL or RCPT commands or other
    commands that require mailbox names.
 
    Note that the backslash, "\", is a quote character, which is used to
    indicate that the next character is to be used literally (instead of
    its normal interpretation).  For example, "Joe\,Smith" indicates a
    single nine character user field with the comma being the fourth
    character of the field.
 
 
 
 
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    To promote interoperability and consistent with long-standing
    guidance about conservative use of the DNS in naming and applications
    (e.g., see section 2.3.1 of the base DNS document, RFC1035 [22]),
    characters outside the set of alphas, digits, and hyphen MUST NOT
    appear in domain name labels for SMTP clients or servers.  In
    particular, the underscore character is not permitted.  SMTP servers
    that receive a command in which invalid character codes have been
    employed, and for which there are no other reasons for rejection,
    MUST reject that command with a 501 response.
 
 4.1.3 Address Literals
 
    Sometimes a host is not known to the domain name system and
    communication (and, in particular, communication to report and repair
    the error) is blocked.  To bypass this barrier a special literal form
    of the address is allowed as an alternative to a domain name.  For
    IPv4 addresses, this form uses four small decimal integers separated
    by dots and enclosed by brackets such as [123.255.37.2], which
    indicates an (IPv4) Internet Address in sequence-of-octets form.  For
    IPv6 and other forms of addressing that might eventually be
    standardized, the form consists of a standardized "tag" that
    identifies the address syntax, a colon, and the address itself, in a
    format specified as part of the IPv6 standards [17].
 
    Specifically:
 
       IPv4-address-literal = Snum 3("." Snum)
       IPv6-address-literal = "IPv6:" IPv6-addr
       General-address-literal = Standardized-tag ":" 1*dcontent
       Standardized-tag = Ldh-str
             ; MUST be specified in a standards-track RFC
             ; and registered with IANA
 
       Snum = 1*3DIGIT  ; representing a decimal integer
             ; value in the range 0 through 255
       Let-dig = ALPHA / DIGIT
       Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
 
       IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
       IPv6-hex  = 1*4HEXDIG
       IPv6-full = IPv6-hex 7(":" IPv6-hex)
       IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":"
                  IPv6-hex)]
             ; The "::" represents at least 2 16-bit groups of zeros
             ; No more than 6 groups in addition to the "::" may be
             ; present
       IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
       IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"
 
 
 
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 RFC 2821             Simple Mail Transfer Protocol            April 2001
 
 
                    [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal
             ; The "::" represents at least 2 16-bit groups of zeros
             ; No more than 4 groups in addition to the "::" and
             ; IPv4-address-literal may be present
 
 4.1.4 Order of Commands
 
    There are restrictions on the order in which these commands may be
    used.
 
    A session that will contain mail transactions MUST first be
    initialized by the use of the EHLO command.  An SMTP server SHOULD
    accept commands for non-mail transactions (e.g., VRFY or EXPN)
    without this initialization.
 
    An EHLO command MAY be issued by a client later in the session.  If
    it is issued after the session begins, the SMTP server MUST clear all
    buffers and reset the state exactly as if a RSET command had been
    issued.  In other words, the sequence of RSET followed immediately by
    EHLO is redundant, but not harmful other than in the performance cost
    of executing unnecessary commands.
 
    If the EHLO command is not acceptable to the SMTP server, 501, 500,
    or 502 failure replies MUST be returned as appropriate.  The SMTP
    server MUST stay in the same state after transmitting these replies
    that it was in before the EHLO was received.
 
    The SMTP client MUST, if possible, ensure that the domain parameter
    to the EHLO command is a valid principal host name (not a CNAME or MX
    name) for its host.  If this is not possible (e.g., when the client's
    address is dynamically assigned and the client does not have an
    obvious name), an address literal SHOULD be substituted for the
    domain name and supplemental information provided that will assist in
    identifying the client.
 
    An SMTP server MAY verify that the domain name parameter in the EHLO
    command actually corresponds to the IP address of the client.
    However, the server MUST NOT refuse to accept a message for this
    reason if the verification fails: the information about verification
    failure is for logging and tracing only.
 
    The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
    during a session, or without previously initializing a session.  SMTP
    servers SHOULD process these normally (that is, not return a 503
    code) even if no EHLO command has yet been received; clients SHOULD
    open a session with EHLO before sending these commands.
 
 
 
 
 
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    If these rules are followed, the example in RFC 821 that shows "550
    access denied to you" in response to an EXPN command is incorrect
    unless an EHLO command precedes the EXPN or the denial of access is
    based on the client's IP address or other authentication or
    authorization-determining mechanisms.
 
    The MAIL command (or the obsolete SEND, SOML, or SAML commands)
    begins a mail transaction.  Once started, a mail transaction consists
    of a transaction beginning command, one or more RCPT commands, and a
    DATA command, in that order.  A mail transaction may be aborted by
    the RSET (or a new EHLO) command.  There may be zero or more
    transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be
    sent if a mail transaction is already open, i.e., it should be sent
    only if no mail transaction had been started in the session, or it
    the previous one successfully concluded with a successful DATA
    command, or if the previous one was aborted with a RSET.
 
    If the transaction beginning command argument is not acceptable, a
    501 failure reply MUST be returned and the SMTP server MUST stay in
    the same state.  If the commands in a transaction are out of order to
    the degree that they cannot be processed by the server, a 503 failure
    reply MUST be returned and the SMTP server MUST stay in the same
    state.
 
    The last command in a session MUST be the QUIT command.  The QUIT
    command cannot be used at any other time in a session, but SHOULD be
    used by the client SMTP to request connection closure, even when no
    session opening command was sent and accepted.
 
 4.1.5 Private-use Commands
 
    As specified in section 2.2.2, commands starting in "X" may be used
    by bilateral agreement between the client (sending) and server
    (receiving) SMTP agents.  An SMTP server that does not recognize such
    a command is expected to reply with "500 Command not recognized".  An
    extended SMTP server MAY list the feature names associated with these
    private commands in the response to the EHLO command.
 
    Commands sent or accepted by SMTP systems that do not start with "X"
    MUST conform to the requirements of section 2.2.2.
 
 4.2 SMTP Replies
 
    Replies to SMTP commands serve to ensure the synchronization of
    requests and actions in the process of mail transfer and to guarantee
    that the SMTP client always knows the state of the SMTP server.
    Every command MUST generate exactly one reply.
 
 
 
 
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    The details of the command-reply sequence are described in section
    4.3.
 
    An SMTP reply consists of a three digit number (transmitted as three
    numeric characters) followed by some text unless specified otherwise
    in this document.  The number is for use by automata to determine
    what state to enter next; the text is for the human user.  The three
    digits contain enough encoded information that the SMTP client need
    not examine the text and may either discard it or pass it on to the
    user, as appropriate.  Exceptions are as noted elsewhere in this
    document.  In particular, the 220, 221, 251, 421, and 551 reply codes
    are associated with message text that must be parsed and interpreted
    by machines.  In the general case, the text may be receiver dependent
    and context dependent, so there are likely to be varying texts for
    each reply code.  A discussion of the theory of reply codes is given
    in section 4.2.1.  Formally, a reply is defined to be the sequence: a
    three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
    reply (as defined in section 4.2.1).  Since, in violation of this
    specification, the text is sometimes not sent, clients which do not
    receive it SHOULD be prepared to process the code alone (with or
    without a trailing space character).  Only the EHLO, EXPN, and HELP
    commands are expected to result in multiline replies in normal
    circumstances, however, multiline replies are allowed for any
    command.
 
    In ABNF, server responses are:
 
       Greeting = "220 " Domain [ SP text ] CRLF
       Reply-line = Reply-code [ SP text ] CRLF
 
    where "Greeting" appears only in the 220 response that announces that
    the server is opening its part of the connection.
 
    An SMTP server SHOULD send only the reply codes listed in this
    document.  An SMTP server SHOULD use the text shown in the examples
    whenever appropriate.
 
    An SMTP client MUST determine its actions only by the reply code, not
    by the text (except for the "change of address" 251 and 551 and, if
    necessary, 220, 221, and 421 replies); in the general case, any text,
    including no text at all (although senders SHOULD NOT send bare
    codes), MUST be acceptable.  The space (blank) following the reply
    code is considered part of the text.  Whenever possible, a receiver-
    SMTP SHOULD test the first digit (severity indication) of the reply
    code.
 
 
 
 
 
 
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    The list of codes that appears below MUST NOT be construed as
    permanent.  While the addition of new codes should be a rare and
    significant activity, with supplemental information in the textual
    part of the response being preferred, new codes may be added as the
    result of new Standards or Standards-track specifications.
    Consequently, a sender-SMTP MUST be prepared to handle codes not
    specified in this document and MUST do so by interpreting the first
    digit only.
 
 4.2.1 Reply Code Severities and Theory
 
    The three digits of the reply each have a special significance.  The
    first digit denotes whether the response is good, bad or incomplete.
    An unsophisticated SMTP client, or one that receives an unexpected
    code, will be able to determine its next action (proceed as planned,
    redo, retrench, etc.) by examining this first digit.  An SMTP client
    that wants to know approximately what kind of error occurred (e.g.,
    mail system error, command syntax error) may examine the second
    digit.  The third digit and any supplemental information that may be
    present is reserved for the finest gradation of information.
 
    There are five values for the first digit of the reply code:
 
    1yz   Positive Preliminary reply
       The command has been accepted, but the requested action is being
       held in abeyance, pending confirmation of the information in this
       reply.  The SMTP client should send another command specifying
       whether to continue or abort the action.  Note: unextended SMTP
       does not have any commands that allow this type of reply, and so
       does not have continue or abort commands.
 
    2yz   Positive Completion reply
       The requested action has been successfully completed.  A new
       request may be initiated.
 
    3yz   Positive Intermediate reply
       The command has been accepted, but the requested action is being
       held in abeyance, pending receipt of further information.  The
       SMTP client should send another command specifying this
       information.  This reply is used in command sequence groups (i.e.,
       in DATA).
 
    4yz   Transient Negative Completion reply
       The command was not accepted, and the requested action did not
       occur.  However, the error condition is temporary and the action
       may be requested again.  The sender should return to the beginning
       of the command sequence (if any).  It is difficult to assign a
       meaning to "transient" when two different sites (receiver- and
 
 
 
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       sender-SMTP agents) must agree on the interpretation.  Each reply
       in this category might have a different time value, but the SMTP
       client is encouraged to try again.  A rule of thumb to determine
       whether a reply fits into the 4yz or the 5yz category (see below)
       is that replies are 4yz if they can be successful if repeated
       without any change in command form or in properties of the sender
       or receiver (that is, the command is repeated identically and the
       receiver does not put up a new implementation.)
 
    5yz   Permanent Negative Completion reply
       The command was not accepted and the requested action did not
       occur.  The SMTP client is discouraged from repeating the exact
       request (in the same sequence).  Even some "permanent" error
       conditions can be corrected, so the human user may want to direct
       the SMTP client to reinitiate the command sequence by direct
       action at some point in the future (e.g., after the spelling has
       been changed, or the user has altered the account status).
 
    The second digit encodes responses in specific categories:
 
    x0z   Syntax: These replies refer to syntax errors, syntactically
       correct commands that do not fit any functional category, and
       unimplemented or superfluous commands.
 
    x1z   Information:  These are replies to requests for information,
       such as status or help.
 
    x2z   Connections: These are replies referring to the transmission
       channel.
 
    x3z   Unspecified.
 
    x4z   Unspecified.
 
    x5z   Mail system: These replies indicate the status of the receiver
       mail system vis-a-vis the requested transfer or other mail system
       action.
 
    The third digit gives a finer gradation of meaning in each category
    specified by the second digit.  The list of replies illustrates this.
    Each reply text is recommended rather than mandatory, and may even
    change according to the command with which it is associated.  On the
    other hand, the reply codes must strictly follow the specifications
    in this section.  Receiver implementations should not invent new
    codes for slightly different situations from the ones described here,
    but rather adapt codes already defined.
 
 
 
 
 
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    For example, a command such as NOOP, whose successful execution does
    not offer the SMTP client any new information, will return a 250
    reply.  The reply is 502 when the command requests an unimplemented
    non-site-specific action.  A refinement of that is the 504 reply for
    a command that is implemented, but that requests an unimplemented
    parameter.
 
    The reply text may be longer than a single line; in these cases the
    complete text must be marked so the SMTP client knows when it can
    stop reading the reply.  This requires a special format to indicate a
    multiple line reply.
 
    The format for multiline replies requires that every line, except the
    last, begin with the reply code, followed immediately by a hyphen,
    "-" (also known as minus), followed by text.  The last line will
    begin with the reply code, followed immediately by <SP>, optionally
    some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>
    if subsequent text is not sent, but clients MUST be prepared for it
    to be omitted.
 
    For example:
 
       123-First line
       123-Second line
       123-234 text beginning with numbers
       123 The last line
 
    In many cases the SMTP client then simply needs to search for a line
    beginning with the reply code followed by <SP> or <CRLF> and ignore
    all preceding lines.  In a few cases, there is important data for the
    client in the reply "text".  The client will be able to identify
    these cases from the current context.
 
 4.2.2 Reply Codes by Function Groups
 
       500 Syntax error, command unrecognized
          (This may include errors such as command line too long)
       501 Syntax error in parameters or arguments
       502 Command not implemented  (see section 4.2.4)
       503 Bad sequence of commands
       504 Command parameter not implemented
 
       211 System status, or system help reply
       214 Help message
          (Information on how to use the receiver or the meaning of a
          particular non-standard command; this reply is useful only
          to the human user)
 
 
 
 
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       220 <domain> Service ready
       221 <domain> Service closing transmission channel
       421 <domain> Service not available, closing transmission channel
          (This may be a reply to any command if the service knows it
          must shut down)
 
       250 Requested mail action okay, completed
       251 User not local; will forward to <forward-path>
          (See section 3.4)
       252 Cannot VRFY user, but will accept message and attempt
           delivery
          (See section 3.5.3)
       450 Requested mail action not taken: mailbox unavailable
          (e.g., mailbox busy)
       550 Requested action not taken: mailbox unavailable
          (e.g., mailbox not found, no access, or command rejected
          for policy reasons)
       451 Requested action aborted: error in processing
       551 User not local; please try <forward-path>
          (See section 3.4)
       452 Requested action not taken: insufficient system storage
       552 Requested mail action aborted: exceeded storage allocation
       553 Requested action not taken: mailbox name not allowed
          (e.g., mailbox syntax incorrect)
       354 Start mail input; end with <CRLF>.<CRLF>
       554 Transaction failed (Or, in the case of a connection-opening
           response, "No SMTP service here")
 
 4.2.3  Reply Codes in Numeric Order
 
       211 System status, or system help reply
       214 Help message
          (Information on how to use the receiver or the meaning of a
          particular non-standard command; this reply is useful only
          to the human user)
       220 <domain> Service ready
       221 <domain> Service closing transmission channel
       250 Requested mail action okay, completed
       251 User not local; will forward to <forward-path>
          (See section 3.4)
       252 Cannot VRFY user, but will accept message and attempt
          delivery
          (See section 3.5.3)
 
       354 Start mail input; end with <CRLF>.<CRLF>
 
 
 
 
 
 
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       421 <domain> Service not available, closing transmission channel
          (This may be a reply to any command if the service knows it
          must shut down)
       450 Requested mail action not taken: mailbox unavailable
          (e.g., mailbox busy)
       451 Requested action aborted: local error in processing
       452 Requested action not taken: insufficient system storage
       500 Syntax error, command unrecognized
          (This may include errors such as command line too long)
       501 Syntax error in parameters or arguments
       502 Command not implemented (see section 4.2.4)
       503 Bad sequence of commands
       504 Command parameter not implemented
       550 Requested action not taken: mailbox unavailable
          (e.g., mailbox not found, no access, or command rejected
          for policy reasons)
       551 User not local; please try <forward-path>
          (See section 3.4)
       552 Requested mail action aborted: exceeded storage allocation
       553 Requested action not taken: mailbox name not allowed
          (e.g., mailbox syntax incorrect)
       554 Transaction failed  (Or, in the case of a connection-opening
           response, "No SMTP service here")
 
 4.2.4 Reply Code 502
 
    Questions have been raised as to when reply code 502 (Command not
    implemented) SHOULD be returned in preference to other codes.  502
    SHOULD be used when the command is actually recognized by the SMTP
    server, but not implemented.  If the command is not recognized, code
    500 SHOULD be returned.  Extended SMTP systems MUST NOT list
    capabilities in response to EHLO for which they will return 502 (or
    500) replies.
 
 4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
 
    When an SMTP server returns a positive completion status (2yz code)
    after the DATA command is completed with <CRLF>.<CRLF>, it accepts
    responsibility for:
 
    -  delivering the message (if the recipient mailbox exists), or
 
    -  if attempts to deliver the message fail due to transient
       conditions, retrying delivery some reasonable number of times at
       intervals as specified in section 4.5.4.
 
 
 
 
 
 
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    -  if attempts to deliver the message fail due to permanent
       conditions, or if repeated attempts to deliver the message fail
       due to transient conditions, returning appropriate notification to
       the sender of the original message (using the address in the SMTP
       MAIL command).
 
    When an SMTP server returns a permanent error status (5yz) code after
    the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
    any subsequent attempt to deliver that message.  The SMTP client
    retains responsibility for delivery of that message and may either
    return it to the user or requeue it for a subsequent attempt (see
    section 4.5.4.1).
 
    The user who originated the message SHOULD be able to interpret the
    return of a transient failure status (by mail message or otherwise)
    as a non-delivery indication, just as a permanent failure would be
    interpreted.  I.e., if the client SMTP successfully handles these
    conditions, the user will not receive such a reply.
 
    When an SMTP server returns a permanent error status (5yz) code after
    the DATA command is completely with <CRLF>.<CRLF>, it MUST NOT make
    any subsequent attempt to deliver the message.  As with temporary
    error status codes, the SMTP client retains responsibility for the
    message, but SHOULD not again attempt delivery to the same server
    without user review and intervention of the message.
 
 4.3 Sequencing of Commands and Replies
 
 4.3.1 Sequencing Overview
 
    The communication between the sender and receiver is an alternating
    dialogue, controlled by the sender.  As such, the sender issues a
    command and the receiver responds with a reply.  Unless other
    arrangements are negotiated through service extensions, the sender
    MUST wait for this response before sending further commands.
 
    One important reply is the connection greeting.  Normally, a receiver
    will send a 220 "Service ready" reply when the connection is
    completed.  The sender SHOULD wait for this greeting message before
    sending any commands.
 
    Note: all the greeting-type replies have the official name (the
    fully-qualified primary domain name) of the server host as the first
    word following the reply code.  Sometimes the host will have no
    meaningful name.  See 4.1.3 for a discussion of alternatives in these
    situations.
 
 
 
 
 
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    For example,
 
       220 ISIF.USC.EDU Service ready
    or
       220 mail.foo.com SuperSMTP v 6.1.2 Service ready
    or
       220 [10.0.0.1] Clueless host service ready
 
    The table below lists alternative success and failure replies for
    each command.  These SHOULD be strictly adhered to: a receiver may
    substitute text in the replies, but the meaning and action implied by
    the code numbers and by the specific command reply sequence cannot be
    altered.
 
 4.3.2 Command-Reply Sequences
 
    Each command is listed with its usual possible replies.  The prefixes
    used before the possible replies are "I" for intermediate, "S" for
    success, and "E" for error.  Since some servers may generate other
    replies under special circumstances, and to allow for future
    extension, SMTP clients SHOULD, when possible, interpret only the
    first digit of the reply and MUST be prepared to deal with
    unrecognized reply codes by interpreting the first digit only.
    Unless extended using the mechanisms described in section 2.2, SMTP
    servers MUST NOT transmit reply codes to an SMTP client that are
    other than three digits or that do not start in a digit between 2 and
    5 inclusive.
 
    These sequencing rules and, in principle, the codes themselves, can
    be extended or modified by SMTP extensions offered by the server and
    accepted (requested) by the client.
 
    In addition to the codes listed below, any SMTP command can return
    any of the following codes if the corresponding unusual circumstances
    are encountered:
 
    500  For the "command line too long" case or if the command name was
       not recognized.  Note that producing a "command not recognized"
       error in response to the required subset of these commands is a
       violation of this specification.
 
    501  Syntax error in command or arguments.  In order to provide for
       future extensions, commands that are specified in this document as
       not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
       message if arguments are supplied in the absence of EHLO-
       advertised extensions.
 
    421  Service shutting down and closing transmission channel
 
 
 
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    Specific sequences are:
 
    CONNECTION ESTABLISHMENT
       S: 220
       E: 554
    EHLO or HELO
       S: 250
       E: 504, 550
    MAIL
       S: 250
       E: 552, 451, 452, 550, 553, 503
    RCPT
       S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
       E: 550, 551, 552, 553, 450, 451, 452, 503, 550
    DATA
       I: 354 -> data -> S: 250
                         E: 552, 554, 451, 452
       E: 451, 554, 503
    RSET
       S: 250
    VRFY
       S: 250, 251, 252
       E: 550, 551, 553, 502, 504
    EXPN
       S: 250, 252
       E: 550, 500, 502, 504
    HELP
       S: 211, 214
       E: 502, 504
    NOOP
       S: 250
    QUIT
       S: 221
 
 4.4 Trace Information
 
    When an SMTP server receives a message for delivery or further
    processing, it MUST insert trace ("time stamp" or "Received")
    information at the beginning of the message content, as discussed in
    section 4.1.1.4.
 
    This line MUST be structured as follows:
 
    -  The FROM field, which MUST be supplied in an SMTP environment,
       SHOULD contain both (1) the name of the source host as presented
       in the EHLO command and (2) an address literal containing the IP
       address of the source, determined from the TCP connection.
 
 
 
 
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    -  The ID field MAY contain an "@" as suggested in RFC 822, but this
       is not required.
 
    -  The FOR field MAY contain a list of <path> entries when multiple
       RCPT commands have been given.  This may raise some security
       issues and is usually not desirable; see section 7.2.
 
    An Internet mail program MUST NOT change a Received: line that was
    previously added to the message header.  SMTP servers MUST prepend
    Received lines to messages; they MUST NOT change the order of
    existing lines or insert Received lines in any other location.
 
    As the Internet grows, comparability of Received fields is important
    for detecting problems, especially slow relays.  SMTP servers that
    create Received fields SHOULD use explicit offsets in the dates
    (e.g., -0800), rather than time zone names of any type.  Local time
    (with an offset) is preferred to UT when feasible.  This formulation
    allows slightly more information about local circumstances to be
    specified.  If UT is needed, the receiver need merely do some simple
    arithmetic to convert the values.  Use of UT loses information about
    the time zone-location of the server.  If it is desired to supply a
    time zone name, it SHOULD be included in a comment.
 
    When the delivery SMTP server makes the "final delivery" of a
    message, it inserts a return-path line at the beginning of the mail
    data.  This use of return-path is required; mail systems MUST support
    it.  The return-path line preserves the information in the <reverse-
    path> from the MAIL command.  Here, final delivery means the message
    has left the SMTP environment.  Normally, this would mean it had been
    delivered to the destination user or an associated mail drop, but in
    some cases it may be further processed and transmitted by another
    mail system.
 
    It is possible for the mailbox in the return path to be different
    from the actual sender's mailbox, for example, if error responses are
    to be delivered to a special error handling mailbox rather than to
    the message sender.  When mailing lists are involved, this
    arrangement is common and useful as a means of directing errors to
    the list maintainer rather than the message originator.
 
    The text above implies that the final mail data will begin with a
    return path line, followed by one or more time stamp lines.  These
    lines will be followed by the mail data headers and body [32].
 
    It is sometimes difficult for an SMTP server to determine whether or
    not it is making final delivery since forwarding or other operations
    may occur after the message is accepted for delivery.  Consequently,
 
 
 
 
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    any further (forwarding, gateway, or relay) systems MAY remove the
    return path and rebuild the MAIL command as needed to ensure that
    exactly one such line appears in a delivered message.
 
    A message-originating SMTP system SHOULD NOT send a message that
    already contains a Return-path header.  SMTP servers performing a
    relay function MUST NOT inspect the message data, and especially not
    to the extent needed to determine if Return-path headers are present.
    SMTP servers making final delivery MAY remove Return-path headers
    before adding their own.
 
    The primary purpose of the Return-path is to designate the address to
    which messages indicating non-delivery or other mail system failures
    are to be sent.  For this to be unambiguous, exactly one return path
    SHOULD be present when the message is delivered.  Systems using RFC
    822 syntax with non-SMTP transports SHOULD designate an unambiguous
    address, associated with the transport envelope, to which error
    reports (e.g., non-delivery messages) should be sent.
 
    Historical note: Text in RFC 822 that appears to contradict the use
    of the Return-path header (or the envelope reverse path address from
    the MAIL command) as the destination for error messages is not
    applicable on the Internet.  The reverse path address (as copied into
    the Return-path) MUST be used as the target of any mail containing
    delivery error messages.
 
    In particular:
 
    -  a gateway from SMTP->elsewhere SHOULD insert a return-path header,
       unless it is known that the "elsewhere" transport also uses
       Internet domain addresses and maintains the envelope sender
       address separately.
 
    -  a gateway from elsewhere->SMTP SHOULD delete any return-path
       header present in the message, and either copy that information to
       the SMTP envelope or combine it with information present in the
       envelope of the other transport system to construct the reverse
       path argument to the MAIL command in the SMTP envelope.
 
    The server must give special treatment to cases in which the
    processing following the end of mail data indication is only
    partially successful.  This could happen if, after accepting several
    recipients and the mail data, the SMTP server finds that the mail
    data could be successfully delivered to some, but not all, of the
    recipients.  In such cases, the response to the DATA command MUST be
    an OK reply.  However, the SMTP server MUST compose and send an
    "undeliverable mail" notification message to the originator of the
    message.
 
 
 
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    A single notification listing all of the failed recipients or
    separate notification messages MUST be sent for each failed
    recipient.  For economy of processing by the sender, the former is
    preferred when possible.  All undeliverable mail notification
    messages are sent using the MAIL command (even if they result from
    processing the obsolete SEND, SOML, or SAML commands) and use a null
    return path as discussed in section 3.7.
 
    The time stamp line and the return path line are formally defined as
    follows:
 
 Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
 
 Time-stamp-line = "Received:" FWS Stamp <CRLF>
 
 Stamp = From-domain By-domain Opt-info ";"  FWS date-time
 
       ; where "date-time" is as defined in [32]
       ; but the "obs-" forms, especially two-digit
       ; years, are prohibited in SMTP and MUST NOT be used.
 
 From-domain = "FROM" FWS Extended-Domain CFWS
 
 By-domain = "BY" FWS Extended-Domain CFWS
 
 Extended-Domain = Domain /
            ( Domain FWS "(" TCP-info ")" ) /
            ( Address-literal FWS "(" TCP-info ")" )
 
 TCP-info = Address-literal / ( Domain FWS Address-literal )
       ; Information derived by server from TCP connection
       ; not client EHLO.
 
 Opt-info = [Via] [With] [ID] [For]
 
 Via = "VIA" FWS Link CFWS
 
 With = "WITH" FWS Protocol CFWS
 
 ID = "ID" FWS String / msg-id CFWS
 
 For = "FOR" FWS 1*( Path / Mailbox ) CFWS
 
 Link = "TCP" / Addtl-Link
 Addtl-Link = Atom
       ; Additional standard names for links are registered with the
          ; Internet Assigned Numbers Authority (IANA).  "Via" is
          ; primarily of value with non-Internet transports.  SMTP
 
 
 
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          ; servers SHOULD NOT use unregistered names.
 Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
 Attdl-Protocol = Atom
       ; Additional standard names for protocols are registered with the
          ; Internet Assigned Numbers Authority (IANA).  SMTP servers
          ; SHOULD NOT use unregistered names.
 
 4.5 Additional Implementation Issues
 
 4.5.1 Minimum Implementation
 
    In order to make SMTP workable, the following minimum implementation
    is required for all receivers.  The following commands MUST be
    supported to conform to this specification:
 
       EHLO
       HELO
       MAIL
       RCPT
       DATA
       RSET
       NOOP
       QUIT
       VRFY
 
    Any system that includes an SMTP server supporting mail relaying or
    delivery MUST support the reserved mailbox "postmaster" as a case-
    insensitive local name.  This postmaster address is not strictly
    necessary if the server always returns 554 on connection opening (as
    described in section 3.1).  The requirement to accept mail for
    postmaster implies that RCPT commands which specify a mailbox for
    postmaster at any of the domains for which the SMTP server provides
    mail service, as well as the special case of "RCPT TO:<Postmaster>"
    (with no domain specification), MUST be supported.
 
    SMTP systems are expected to make every reasonable effort to accept
    mail directed to Postmaster from any other system on the Internet.
    In extreme cases --such as to contain a denial of service attack or
    other breach of security-- an SMTP server may block mail directed to
    Postmaster.  However, such arrangements SHOULD be narrowly tailored
    so as to avoid blocking messages which are not part of such attacks.
 
 4.5.2 Transparency
 
    Without some provision for data transparency, the character sequence
    "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
    In general, users are not aware of such "forbidden" sequences.  To
 
 
 
 
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    allow all user composed text to be transmitted transparently, the
    following procedures are used:
 
    -  Before sending a line of mail text, the SMTP client checks the
       first character of the line.  If it is a period, one additional
       period is inserted at the beginning of the line.
 
    -  When a line of mail text is received by the SMTP server, it checks
       the line.  If the line is composed of a single period, it is
       treated as the end of mail indicator.  If the first character is a
       period and there are other characters on the line, the first
       character is deleted.
 
    The mail data may contain any of the 128 ASCII characters.  All
    characters are to be delivered to the recipient's mailbox, including
    spaces, vertical and horizontal tabs, and other control characters.
    If the transmission channel provides an 8-bit byte (octet) data
    stream, the 7-bit ASCII codes are transmitted right justified in the
    octets, with the high order bits cleared to zero.  See 3.7 for
    special treatment of these conditions in SMTP systems serving a relay
    function.
 
    In some systems it may be necessary to transform the data as it is
    received and stored.  This may be necessary for hosts that use a
    different character set than ASCII as their local character set, that
    store data in records rather than strings, or which use special
    character sequences as delimiters inside mailboxes.  If such
    transformations are necessary, they MUST be reversible, especially if
    they are applied to mail being relayed.
 
 4.5.3 Sizes and Timeouts
 
 4.5.3.1 Size limits and minimums
 
    There are several objects that have required minimum/maximum sizes.
    Every implementation MUST be able to receive objects of at least
    these sizes.  Objects larger than these sizes SHOULD be avoided when
    possible.  However, some Internet mail constructs such as encoded
    X.400 addresses [16] will often require larger objects: clients MAY
    attempt to transmit these, but MUST be prepared for a server to
    reject them if they cannot be handled by it.  To the maximum extent
    possible, implementation techniques which impose no limits on the
    length of these objects should be used.
 
    local-part
       The maximum total length of a user name or other local-part is 64
       characters.
 
 
 
 
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    domain
       The maximum total length of a domain name or number is 255
       characters.
 
    path
       The maximum total length of a reverse-path or forward-path is 256
       characters (including the punctuation and element separators).
 
    command line
       The maximum total length of a command line including the command
       word and the <CRLF> is 512 characters.  SMTP extensions may be
       used to increase this limit.
 
    reply line
       The maximum total length of a reply line including the reply code
       and the <CRLF> is 512 characters.  More information may be
       conveyed through multiple-line replies.
 
    text line
       The maximum total length of a text line including the <CRLF> is
       1000 characters (not counting the leading dot duplicated for
       transparency).  This number may be increased by the use of SMTP
       Service Extensions.
 
    message content
       The maximum total length of a message content (including any
       message headers as well as the message body) MUST BE at least 64K
       octets.  Since the introduction of Internet standards for
       multimedia mail [12], message lengths on the Internet have grown
       dramatically, and message size restrictions should be avoided if
       at all possible.  SMTP server systems that must impose
       restrictions SHOULD implement the "SIZE" service extension [18],
       and SMTP client systems that will send large messages SHOULD
       utilize it when possible.
 
    recipients buffer
       The minimum total number of recipients that must be buffered is
       100 recipients.  Rejection of messages (for excessive recipients)
       with fewer than 100 RCPT commands is a violation of this
       specification.  The general principle that relaying SMTP servers
       MUST NOT, and delivery SMTP servers SHOULD NOT, perform validation
       tests on message headers suggests that rejecting a message based
       on the total number of recipients shown in header fields is to be
       discouraged.  A server which imposes a limit on the number of
       recipients MUST behave in an orderly fashion,  such as to reject
       additional addresses over its limit rather than silently
       discarding addresses previously accepted.  A client that needs to
 
 
 
 
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       deliver a message containing over 100 RCPT commands SHOULD be
       prepared to transmit in 100-recipient "chunks" if the server
       declines to accept more than 100 recipients in a single message.
 
    Errors due to exceeding these limits may be reported by using the
    reply codes.  Some examples of reply codes are:
 
       500 Line too long.
    or
       501 Path too long
    or
       452 Too many recipients  (see below)
    or
       552 Too much mail data.
 
    RFC 821 [30] incorrectly listed the error where an SMTP server
    exhausts its implementation limit on the number of RCPT commands
    ("too many recipients") as having reply code 552.  The correct reply
    code for this condition is 452.  Clients SHOULD treat a 552 code in
    this case as a temporary, rather than permanent, failure so the logic
    below works.
 
    When a conforming SMTP server encounters this condition, it has at
    least 100 successful RCPT commands in its recipients buffer.  If the
    server is able to accept the message, then at least these 100
    addresses will be removed from the SMTP client's queue.  When the
    client attempts retransmission of those addresses which received 452
    responses, at least 100 of these will be able to fit in the SMTP
    server's recipients buffer.  Each retransmission attempt which is
    able to deliver anything will be able to dispose of at least 100 of
    these recipients.
 
    If an SMTP server has an implementation limit on the number of RCPT
    commands and this limit is exhausted, it MUST use a response code of
    452 (but the client SHOULD also be prepared for a 552, as noted
    above).  If the server has a configured site-policy limitation on the
    number of RCPT commands, it MAY instead use a 5XX response code.
    This would be most appropriate if the policy limitation was intended
    to apply if the total recipient count for a particular message body
    were enforced even if that message body was sent in multiple mail
    transactions.
 
 4.5.3.2 Timeouts
 
    An SMTP client MUST provide a timeout mechanism.  It MUST use per-
    command timeouts rather than somehow trying to time the entire mail
    transaction.  Timeouts SHOULD be easily reconfigurable, preferably
    without recompiling the SMTP code.  To implement this, a timer is set
 
 
 
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    for each SMTP command and for each buffer of the data transfer.  The
    latter means that the overall timeout is inherently proportional to
    the size of the message.
 
    Based on extensive experience with busy mail-relay hosts, the minimum
    per-command timeout values SHOULD be as follows:
 
    Initial 220 Message: 5 minutes
       An SMTP client process needs to distinguish between a failed TCP
       connection and a delay in receiving the initial 220 greeting
       message.  Many SMTP servers accept a TCP connection but delay
       delivery of the 220 message until their system load permits more
       mail to be processed.
 
    MAIL Command: 5 minutes
 
    RCPT Command: 5 minutes
       A longer timeout is required if processing of mailing lists and
       aliases is not deferred until after the message was accepted.
 
    DATA Initiation: 2 minutes
       This is while awaiting the "354 Start Input" reply to a DATA
       command.
 
    Data Block: 3 minutes
       This is while awaiting the completion of each TCP SEND call
       transmitting a chunk of data.
 
    DATA Termination: 10 minutes.
       This is while awaiting the "250 OK" reply.  When the receiver gets
       the final period terminating the message data, it typically
       performs processing to deliver the message to a user mailbox.  A
       spurious timeout at this point would be very wasteful and would
       typically result in delivery of multiple copies of the message,
       since it has been successfully sent and the server has accepted
       responsibility for delivery.  See section 6.1 for additional
       discussion.
 
    An SMTP server SHOULD have a timeout of at least 5 minutes while it
    is awaiting the next command from the sender.
 
 4.5.4 Retry Strategies
 
    The common structure of a host SMTP implementation includes user
    mailboxes, one or more areas for queuing messages in transit, and one
    or more daemon processes for sending and receiving mail.  The exact
    structure will vary depending on the needs of the users on the host
 
 
 
 
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    and the number and size of mailing lists supported by the host.  We
    describe several optimizations that have proved helpful, particularly
    for mailers supporting high traffic levels.
 
    Any queuing strategy MUST include timeouts on all activities on a
    per-command basis.  A queuing strategy MUST NOT send error messages
    in response to error messages under any circumstances.
 
 4.5.4.1 Sending Strategy
 
    The general model for an SMTP client is one or more processes that
    periodically attempt to transmit outgoing mail.  In a typical system,
    the program that composes a message has some method for requesting
    immediate attention for a new piece of outgoing mail, while mail that
    cannot be transmitted immediately MUST be queued and periodically
    retried by the sender.  A mail queue entry will include not only the
    message itself but also the envelope information.
 
    The sender MUST delay retrying a particular destination after one
    attempt has failed.  In general, the retry interval SHOULD be at
    least 30 minutes; however, more sophisticated and variable strategies
    will be beneficial when the SMTP client can determine the reason for
    non-delivery.
 
    Retries continue until the message is transmitted or the sender gives
    up; the give-up time generally needs to be at least 4-5 days.  The
    parameters to the retry algorithm MUST be configurable.
 
    A client SHOULD keep a list of hosts it cannot reach and
    corresponding connection timeouts, rather than just retrying queued
    mail items.
 
    Experience suggests that failures are typically transient (the target
    system or its connection has crashed), favoring a policy of two
    connection attempts in the first hour the message is in the queue,
    and then backing off to one every two or three hours.
 
    The SMTP client can shorten the queuing delay in cooperation with the
    SMTP server.  For example, if mail is received from a particular
    address, it is likely that mail queued for that host can now be sent.
    Application of this principle may, in many cases, eliminate the
    requirement for an explicit "send queues now" function such as ETRN
    [9].
 
    The strategy may be further modified as a result of multiple
    addresses per host (see below) to optimize delivery time vs. resource
    usage.
 
 
 
 
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    An SMTP client may have a large queue of messages for each
    unavailable destination host.  If all of these messages were retried
    in every retry cycle, there would be excessive Internet overhead and
    the sending system would be blocked for a long period.  Note that an
    SMTP client can generally determine that a delivery attempt has
    failed only after a timeout of several minutes and even a one-minute
    timeout per connection will result in a very large delay if retries
    are repeated for dozens, or even hundreds, of queued messages to the
    same host.
 
    At the same time, SMTP clients SHOULD use great care in caching
    negative responses from servers.  In an extreme case, if EHLO is
    issued multiple times during the same SMTP connection, different
    answers may be returned by the server.  More significantly, 5yz
    responses to the MAIL command MUST NOT be cached.
 
    When a mail message is to be delivered to multiple recipients, and
    the SMTP server to which a copy of the message is to be sent is the
    same for multiple recipients, then only one copy of the message
    SHOULD be transmitted.  That is, the SMTP client SHOULD use the
    command sequence:  MAIL, RCPT, RCPT,... RCPT, DATA instead of the
    sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there
    are very many addresses, a limit on the number of RCPT commands per
    MAIL command MAY be imposed.  Implementation of this efficiency
    feature is strongly encouraged.
 
    Similarly, to achieve timely delivery, the SMTP client MAY support
    multiple concurrent outgoing mail transactions.  However, some limit
    may be appropriate to protect the host from devoting all its
    resources to mail.
 
 4.5.4.2 Receiving Strategy
 
    The SMTP server SHOULD attempt to keep a pending listen on the SMTP
    port at all times.  This requires the support of multiple incoming
    TCP connections for SMTP.  Some limit MAY be imposed but servers that
    cannot handle more than one SMTP transaction at a time are not in
    conformance with the intent of this specification.
 
    As discussed above, when the SMTP server receives mail from a
    particular host address, it could activate its own SMTP queuing
    mechanisms to retry any mail pending for that host address.
 
 4.5.5   Messages with a null reverse-path
 
    There are several types of notification messages which are required
    by existing and proposed standards to be sent with a null reverse
    path, namely non-delivery notifications as discussed in section 3.7,
 
 
 
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    other kinds of Delivery Status Notifications (DSNs) [24], and also
    Message Disposition Notifications (MDNs) [10].  All of these kinds of
    messages are notifications about a previous message, and they are
    sent to the reverse-path of the previous mail message.  (If the
    delivery of such a notification message fails, that usually indicates
    a problem with the mail system of the host to which the notification
    message is addressed.  For this reason, at some hosts the MTA is set
    up to forward such failed notification messages to someone who is
    able to fix problems with the mail system, e.g., via the postmaster
    alias.)
 
    All other types of messages (i.e., any message which is not required
    by a standards-track RFC to have a null reverse-path) SHOULD be sent
    with with a valid, non-null reverse-path.
 
    Implementors of automated email processors should be careful to make
    sure that the various kinds of messages with null reverse-path are
    handled correctly, in particular such systems SHOULD NOT reply to
    messages with null reverse-path.
 
 5. Address Resolution and Mail Handling
 
    Once an SMTP client lexically identifies a domain to which mail will
    be delivered for processing (as described in sections 3.6 and 3.7), a
    DNS lookup MUST be performed to resolve the domain name [22].  The
    names are expected to be fully-qualified domain names (FQDNs):
    mechanisms for inferring FQDNs from partial names or local aliases
    are outside of this specification and, due to a history of problems,
    are generally discouraged.  The lookup first attempts to locate an MX
    record associated with the name.  If a CNAME record is found instead,
    the resulting name is processed as if it were the initial name.  If
    no MX records are found, but an A RR is found, the A RR is treated as
    if it was associated with an implicit MX RR, with a preference of 0,
    pointing to that host.  If one or more MX RRs are found for a given
    name, SMTP systems MUST NOT utilize any A RRs associated with that
    name unless they are located using the MX RRs; the "implicit MX" rule
    above applies only if there are no MX records present.  If MX records
    are present, but none of them are usable, this situation MUST be
    reported as an error.
 
    When the lookup succeeds, the mapping can result in a list of
    alternative delivery addresses rather than a single address, because
    of multiple MX records, multihoming, or both.  To provide reliable
    mail transmission, the SMTP client MUST be able to try (and retry)
    each of the relevant addresses in this list in order, until a
    delivery attempt succeeds.  However, there MAY also be a configurable
    limit on the number of alternate addresses that can be tried.  In any
    case, the SMTP client SHOULD try at least two addresses.
 
 
 
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    Two types of information is used to rank the host addresses: multiple
    MX records, and multihomed hosts.
 
    Multiple MX records contain a preference indication that MUST be used
    in sorting (see below).  Lower numbers are more preferred than higher
    ones.  If there are multiple destinations with the same preference
    and there is no clear reason to favor one (e.g., by recognition of an
    easily-reached address), then the sender-SMTP MUST randomize them to
    spread the load across multiple mail exchangers for a specific
    organization.
 
    The destination host (perhaps taken from the preferred MX record) may
    be multihomed, in which case the domain name resolver will return a
    list of alternative IP addresses.  It is the responsibility of the
    domain name resolver interface to have ordered this list by
    decreasing preference if necessary, and SMTP MUST try them in the
    order presented.
 
    Although the capability to try multiple alternative addresses is
    required, specific installations may want to limit or disable the use
    of alternative addresses.  The question of whether a sender should
    attempt retries using the different addresses of a multihomed host
    has been controversial.  The main argument for using the multiple
    addresses is that it maximizes the probability of timely delivery,
    and indeed sometimes the probability of any delivery; the counter-
    argument is that it may result in unnecessary resource use.  Note
    that resource use is also strongly determined by the sending strategy
    discussed in section 4.5.4.1.
 
    If an SMTP server receives a message with a destination for which it
    is a designated Mail eXchanger, it MAY relay the message (potentially
    after having rewritten the MAIL FROM and/or RCPT TO addresses), make
    final delivery of the message, or hand it off using some mechanism
    outside the SMTP-provided transport environment.  Of course, neither
    of the latter require that the list of MX records be examined
    further.
 
    If it determines that it should relay the message without rewriting
    the address, it MUST sort the MX records to determine candidates for
    delivery.  The records are first ordered by preference, with the
    lowest-numbered records being most preferred.  The relay host MUST
    then inspect the list for any of the names or addresses by which it
    might be known in mail transactions.  If a matching record is found,
    all records at that preference level and higher-numbered ones MUST be
    discarded from consideration.  If there are no records left at that
    point, it is an error condition, and the message MUST be returned as
    undeliverable.  If records do remain, they SHOULD be tried, best
    preference first, as described above.
 
 
 
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 6. Problem Detection and Handling
 
 6.1 Reliable Delivery and Replies by Email
 
    When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
    message in response to DATA), it is accepting responsibility for
    delivering or relaying the message.  It must take this responsibility
    seriously.  It MUST NOT lose the message for frivolous reasons, such
    as because the host later crashes or because of a predictable
    resource shortage.
 
    If there is a delivery failure after acceptance of a message, the
    receiver-SMTP MUST formulate and mail a notification message.  This
    notification MUST be sent using a null ("<>") reverse path in the
    envelope.  The recipient of this notification MUST be the address
    from the envelope return path (or the Return-Path: line).  However,
    if this address is null ("<>"), the receiver-SMTP MUST NOT send a
    notification.  Obviously, nothing in this section can or should
    prohibit local decisions (i.e., as part of the same system
    environment as the receiver-SMTP) to log or otherwise transmit
    information about null address events locally if that is desired.  If
    the address is an explicit source route, it MUST be stripped down to
    its final hop.
 
    For example, suppose that an error notification must be sent for a
    message that arrived with:
 
       MAIL FROM:<@a,@b:user@d>
 
    The notification message MUST be sent using:
 
       RCPT TO:<user@d>
 
    Some delivery failures after the message is accepted by SMTP will be
    unavoidable.  For example, it may be impossible for the receiving
    SMTP server to validate all the delivery addresses in RCPT command(s)
    due to a "soft" domain system error, because the target is a mailing
    list (see earlier discussion of RCPT), or because the server is
    acting as a relay and has no immediate access to the delivering
    system.
 
    To avoid receiving duplicate messages as the result of timeouts, a
    receiver-SMTP MUST seek to minimize the time required to respond to
    the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [28] for
    a discussion of this problem.
 
 
 
 
 
 
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 6.2 Loop Detection
 
    Simple counting of the number of "Received:" headers in a message has
    proven to be an effective, although rarely optimal, method of
    detecting loops in mail systems.  SMTP servers using this technique
    SHOULD use a large rejection threshold, normally at least 100
    Received entries.  Whatever mechanisms are used, servers MUST contain
    provisions for detecting and stopping trivial loops.
 
 6.3 Compensating for Irregularities
 
    Unfortunately, variations, creative interpretations, and outright
    violations of Internet mail protocols do occur; some would suggest
    that they occur quite frequently.  The debate as to whether a well-
    behaved SMTP receiver or relay should reject a malformed message,
    attempt to pass it on unchanged, or attempt to repair it to increase
    the odds of successful delivery (or subsequent reply) began almost
    with the dawn of structured network mail and shows no signs of
    abating.  Advocates of rejection claim that attempted repairs are
    rarely completely adequate and that rejection of bad messages is the
    only way to get the offending software repaired.  Advocates of
    "repair" or "deliver no matter what" argue that users prefer that
    mail go through it if at all possible and that there are significant
    market pressures in that direction.  In practice, these market
    pressures may be more important to particular vendors than strict
    conformance to the standards, regardless of the preference of the
    actual developers.
 
    The problems associated with ill-formed messages were exacerbated by
    the introduction of the split-UA mail reading protocols [3, 26, 5,
    21].  These protocols have encouraged the use of SMTP as a posting
    protocol, and SMTP servers as relay systems for these client hosts
    (which are often only intermittently connected to the Internet).
    Historically, many of those client machines lacked some of the
    mechanisms and information assumed by SMTP (and indeed, by the mail
    format protocol [7]).  Some could not keep adequate track of time;
    others had no concept of time zones; still others could not identify
    their own names or addresses; and, of course, none could satisfy the
    assumptions that underlay RFC 822's conception of authenticated
    addresses.
 
    In response to these weak SMTP clients, many SMTP systems now
    complete messages that are delivered to them in incomplete or
    incorrect form.  This strategy is generally considered appropriate
    when the server can identify or authenticate the client, and there
    are prior agreements between them.  By contrast, there is at best
    great concern about fixes applied by a relay or delivery SMTP server
    that has little or no knowledge of the user or client machine.
 
 
 
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    The following changes to a message being processed MAY be applied
    when necessary by an originating SMTP server, or one used as the
    target of SMTP as an initial posting protocol:
 
    -  Addition of a message-id field when none appears
 
    -  Addition of a date, time or time zone when none appears
 
    -  Correction of addresses to proper FQDN format
 
    The less information the server has about the client, the less likely
    these changes are to be correct and the more caution and conservatism
    should be applied when considering whether or not to perform fixes
    and how.  These changes MUST NOT be applied by an SMTP server that
    provides an intermediate relay function.
 
    In all cases, properly-operating clients supplying correct
    information are preferred to corrections by the SMTP server.  In all
    cases, documentation of actions performed by the servers (in trace
    fields and/or header comments) is strongly encouraged.
 
 7. Security Considerations
 
 7.1 Mail Security and Spoofing
 
    SMTP mail is inherently insecure in that it is feasible for even
    fairly casual users to negotiate directly with receiving and relaying
    SMTP servers and create messages that will trick a naive recipient
    into believing that they came from somewhere else.  Constructing such
    a message so that the "spoofed" behavior cannot be detected by an
    expert is somewhat more difficult, but not sufficiently so as to be a
    deterrent to someone who is determined and knowledgeable.
    Consequently, as knowledge of Internet mail increases, so does the
    knowledge that SMTP mail inherently cannot be authenticated, or
    integrity checks provided, at the transport level.  Real mail
    security lies only in end-to-end methods involving the message
    bodies, such as those which use digital signatures (see [14] and,
    e.g., PGP [4] or S/MIME [31]).
 
    Various protocol extensions and configuration options that provide
    authentication at the transport level (e.g., from an SMTP client to
    an SMTP server) improve somewhat on the traditional situation
    described above.  However, unless they are accompanied by careful
    handoffs of responsibility in a carefully-designed trust environment,
    they remain inherently weaker than end-to-end mechanisms which use
    digitally signed messages rather than depending on the integrity of
    the transport system.
 
 
 
 
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    Efforts to make it more difficult for users to set envelope return
    path and header "From" fields to point to valid addresses other than
    their own are largely misguided: they frustrate legitimate
    applications in which mail is sent by one user on behalf of another
    or in which error (or normal) replies should be directed to a special
    address.  (Systems that provide convenient ways for users to alter
    these fields on a per-message basis should attempt to establish a
    primary and permanent mailbox address for the user so that Sender
    fields within the message data can be generated sensibly.)
 
    This specification does not further address the authentication issues
    associated with SMTP other than to advocate that useful functionality
    not be disabled in the hope of providing some small margin of
    protection against an ignorant user who is trying to fake mail.
 
 7.2 "Blind" Copies
 
    Addresses that do not appear in the message headers may appear in the
    RCPT commands to an SMTP server for a number of reasons.  The two
    most common involve the use of a mailing address as a "list exploder"
    (a single address that resolves into multiple addresses) and the
    appearance of "blind copies".  Especially when more than one RCPT
    command is present, and in order to avoid defeating some of the
    purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
    the full set of RCPT command arguments into the headers, either as
    part of trace headers or as informational or private-extension
    headers.  Since this rule is often violated in practice, and cannot
    be enforced, sending SMTP systems that are aware of "bcc" use MAY
    find it helpful to send each blind copy as a separate message
    transaction containing only a single RCPT command.
 
    There is no inherent relationship between either "reverse" (from
    MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
    transaction ("envelope") and the addresses in the headers.  Receiving
    systems SHOULD NOT attempt to deduce such relationships and use them
    to alter the headers of the message for delivery.  The popular
    "Apparently-to" header is a violation of this principle as well as a
    common source of unintended information disclosure and SHOULD NOT be
    used.
 
 7.3 VRFY, EXPN, and Security
 
    As discussed in section 3.5, individual sites may want to disable
    either or both of VRFY or EXPN for security reasons.  As a corollary
    to the above, implementations that permit this MUST NOT appear to
    have verified addresses that are not, in fact, verified.  If a site
 
 
 
 
 
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    disables these commands for security reasons, the SMTP server MUST
    return a 252 response, rather than a code that could be confused with
    successful or unsuccessful verification.
 
    Returning a 250 reply code with the address listed in the VRFY
    command after having checked it only for syntax violates this rule.
    Of course, an implementation that "supports" VRFY by always returning
    550 whether or not the address is valid is equally not in
    conformance.
 
    Within the last few years, the contents of mailing lists have become
    popular as an address information source for so-called "spammers."
    The use of EXPN to "harvest" addresses has increased as list
    administrators have installed protections against inappropriate uses
    of the lists themselves.  Implementations SHOULD still provide
    support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.
    As authentication mechanisms are introduced into SMTP, some sites may
    choose to make EXPN available only to authenticated requestors.
 
 7.4 Information Disclosure in Announcements
 
    There has been an ongoing debate about the tradeoffs between the
    debugging advantages of announcing server type and version (and,
    sometimes, even server domain name) in the greeting response or in
    response to the HELP command and the disadvantages of exposing
    information that might be useful in a potential hostile attack.  The
    utility of the debugging information is beyond doubt.  Those who
    argue for making it available point out that it is far better to
    actually secure an SMTP server rather than hope that trying to
    conceal known vulnerabilities by hiding the server's precise identity
    will provide more protection.  Sites are encouraged to evaluate the
    tradeoff with that issue in mind; implementations are strongly
    encouraged to minimally provide for making type and version
    information available in some way to other network hosts.
 
 7.5 Information Disclosure in Trace Fields
 
    In some circumstances, such as when mail originates from within a LAN
    whose hosts are not directly on the public Internet, trace
    ("Received") fields produced in conformance with this specification
    may disclose host names and similar information that would not
    normally be available.  This ordinarily does not pose a problem, but
    sites with special concerns about name disclosure should be aware of
    it.  Also, the optional FOR clause should be supplied with caution or
    not at all when multiple recipients are involved lest it
    inadvertently disclose the identities of "blind copy" recipients to
    others.
 
 
 
 
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 7.6 Information Disclosure in Message Forwarding
 
    As discussed in section 3.4, use of the 251 or 551 reply codes to
    identify the replacement address associated with a mailbox may
    inadvertently disclose sensitive information.  Sites that are
    concerned about those issues should ensure that they select and
    configure servers appropriately.
 
 7.7 Scope of Operation of SMTP Servers
 
    It is a well-established principle that an SMTP server may refuse to
    accept mail for any operational or technical reason that makes sense
    to the site providing the server.  However, cooperation among sites
    and installations makes the Internet possible.  If sites take
    excessive advantage of the right to reject traffic, the ubiquity of
    email availability (one of the strengths of the Internet) will be
    threatened; considerable care should be taken and balance maintained
    if a site decides to be selective about the traffic it will accept
    and process.
 
    In recent years, use of the relay function through arbitrary sites
    has been used as part of hostile efforts to hide the actual origins
    of mail.  Some sites have decided to limit the use of the relay
    function to known or identifiable sources, and implementations SHOULD
    provide the capability to perform this type of filtering.  When mail
    is rejected for these or other policy reasons, a 550 code SHOULD be
    used in response to EHLO, MAIL, or RCPT as appropriate.
 
 8. IANA Considerations
 
    IANA will maintain three registries in support of this specification.
    The first consists of SMTP service extensions with the associated
    keywords, and, as needed, parameters and verbs.  As specified in
    section 2.2.2, no entry may be made in this registry that starts in
    an "X".  Entries may be made only for service extensions (and
    associated keywords, parameters, or verbs) that are defined in
    standards-track or experimental RFCs specifically approved by the
    IESG for this purpose.
 
    The second registry consists of "tags" that identify forms of domain
    literals other than those for IPv4 addresses (specified in RFC 821
    and in this document) and IPv6 addresses (specified in this
    document).  Additional literal types require standardization before
    being used; none are anticipated at this time.
 
    The third, established by RFC 821 and renewed by this specification,
    is a registry of link and protocol identifiers to be used with the
    "via" and "with" subclauses of the time stamp ("Received: header")
 
 
 
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    described in section 4.4.  Link and protocol identifiers in addition
    to those specified in this document may be registered only by
    standardization or by way of an RFC-documented, IESG-approved,
    Experimental protocol extension.
 
 9. References
 
    [1]  American National Standards Institute (formerly United States of
         America Standards Institute), X3.4, 1968, "USA Code for
         Information Interchange". ANSI X3.4-1968 has been replaced by
         newer versions with slight modifications, but the 1968 version
         remains definitive for the Internet.
 
    [2]  Braden, R., "Requirements for Internet hosts - application and
         support", STD 3, RFC 1123, October 1989.
 
    [3]  Butler, M., Chase, D., Goldberger, J., Postel, J. and J.
         Reynolds, "Post Office Protocol - version 2", RFC 937, February
         1985.
 
    [4]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
         Message Format", RFC 2440, November 1998.
 
    [5]  Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC
         1176, August 1990.
 
    [6]  Crispin, M., "Internet Message Access Protocol - Version 4", RFC
         2060, December 1996.
 
    [7]  Crocker, D., "Standard for the Format of ARPA Internet Text
         Messages", RFC 822, August 1982.
 
    [8]  Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.
 
    [9]  De Winter, J., "SMTP Service Extension for Remote Message Queue
         Starting", RFC 1985, August 1996.
 
    [10] Fajman, R., "An Extensible Message Format for Message
         Disposition Notifications", RFC 2298, March 1998.
 
    [11] Freed, N, "Behavior of and Requirements for Internet Firewalls",
         RFC 2979, October 2000.
 
    [12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, December 1996.
 
 
 
 
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    [13] Freed, N., "SMTP Service Extension for Command Pipelining", RFC
         2920, September 2000.
 
    [14] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security
         Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
         RFC 1847, October 1995.
 
    [15] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
         December 1998.
 
    [16] Kille, S., "Mapping between X.400 and RFC822/MIME", RFC 2156,
         January 1998.
 
    [17] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.
 
    [18] Klensin, J., Freed, N. and K. Moore, "SMTP Service Extension for
         Message Size Declaration", STD 10, RFC 1870, November 1995.
 
    [19] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
         "SMTP Service Extensions", STD 10, RFC 1869, November 1995.
 
    [20] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
         "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July
         1994.
 
    [21] Lambert, M., "PCMAIL: A distributed mail system for personal
         computers", RFC 1056, July 1988.
 
    [22] Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.
 
         Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.
 
    [23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
         Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
         December 1996.
 
    [24] Moore, K., "SMTP Service Extension for Delivery Status
         Notifications", RFC 1891, January 1996.
 
    [25] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
         Delivery Status Notifications", RFC 1894, January 1996.
 
    [26] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD
         53, RFC 1939, May 1996.
 
 
 
 
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    [27] Partridge, C., "Mail routing and the domain system", RFC 974,
         January 1986.
 
    [28] Partridge, C., "Duplicate messages and SMTP", RFC 1047, February
         1988.
 
    [29] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
         Program Protocol Specification", STD 7, RFC 793, September 1981.
 
    [30] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August
         1982.
 
    [31] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
         2633, June 1999.
 
    [32] Resnick, P., Ed., "Internet Message Format", RFC 2822, April
         2001.
 
    [33] Vaudreuil, G., "SMTP Service Extensions for Transmission of
         Large and Binary MIME Messages", RFC 1830, August 1995.
 
    [34] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
         January 1996.
 
 10. Editor's Address
 
    John C. Klensin
    AT&T Laboratories
    99 Bedford St
    Boston, MA 02111 USA
 
    Phone: 617-574-3076
    EMail: klensin@research.att.com
 
 11. Acknowledgments
 
    Many people worked long and hard on the many iterations of this
    document.  There was wide-ranging debate in the IETF DRUMS Working
    Group, both on its mailing list and in face to face discussions,
    about many technical issues and the role of a revised standard for
    Internet mail transport, and many contributors helped form the
    wording in this specification.  The hundreds of participants in the
    many discussions since RFC 821 was produced are too numerous to
    mention, but they all helped this document become what it is.
 
 
 
 
 
 
 
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 APPENDICES
 
 A. TCP Transport Service
 
    The TCP connection supports the transmission of 8-bit bytes.  The
    SMTP data is 7-bit ASCII characters.  Each character is transmitted
    as an 8-bit byte with the high-order bit cleared to zero.  Service
    extensions may modify this rule to permit transmission of full 8-bit
    data bytes as part of the message body, but not in SMTP commands or
    responses.
 
 B. Generating SMTP Commands from RFC 822 Headers
 
    Some systems use RFC 822 headers (only) in a mail submission
    protocol, or otherwise generate SMTP commands from RFC 822 headers
    when such a message is handed to an MTA from a UA.  While the MTA-UA
    protocol is a private matter, not covered by any Internet Standard,
    there are problems with this approach.  For example, there have been
    repeated problems with proper handling of "bcc" copies and
    redistribution lists when information that conceptually belongs to a
    mail envelopes is not separated early in processing from header
    information (and kept separate).
 
    It is recommended that the UA provide its initial ("submission
    client") MTA with an envelope separate from the message itself.
    However, if the envelope is not supplied, SMTP commands SHOULD be
    generated as follows:
 
    1. Each recipient address from a TO, CC, or BCC header field SHOULD
       be copied to a RCPT command (generating multiple message copies if
       that is required for queuing or delivery).  This includes any
       addresses listed in a RFC 822 "group".  Any BCC fields SHOULD then
       be removed from the headers.  Once this process is completed, the
       remaining headers SHOULD be checked to verify that at least one
       To:, Cc:, or Bcc: header remains.  If none do, then a bcc: header
       with no additional information SHOULD be inserted as specified in
       [32].
 
    2. The return address in the MAIL command SHOULD, if possible, be
       derived from the system's identity for the submitting (local)
       user, and the "From:" header field otherwise.  If there is a
       system identity available, it SHOULD also be copied to the Sender
       header field if it is different from the address in the From
       header field.  (Any Sender field that was already there SHOULD be
       removed.)  Systems may provide a way for submitters to override
       the envelope return address, but may want to restrict its use to
       privileged users.  This will not prevent mail forgery, but may
       lessen its incidence; see section 7.1.
 
 
 
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    When an MTA is being used in this way, it bears responsibility for
    ensuring that the message being transmitted is valid.  The mechanisms
    for checking that validity, and for handling (or returning) messages
    that are not valid at the time of arrival, are part of the MUA-MTA
    interface and not covered by this specification.
 
    A submission protocol based on Standard RFC 822 information alone
    MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
    system into an SMTP environment.  Additional information to construct
    an envelope must come from some source in the other environment,
    whether supplemental headers or the foreign system's envelope.
 
    Attempts to gateway messages using only their header "to" and "cc"
    fields have repeatedly caused mail loops and other behavior adverse
    to the proper functioning of the Internet mail environment.  These
    problems have been especially common when the message originates from
    an Internet mailing list and is distributed into the foreign
    environment using envelope information.  When these messages are then
    processed by a header-only remailer, loops back to the Internet
    environment (and the mailing list) are almost inevitable.
 
 C. Source Routes
 
    Historically, the <reverse-path> was a reverse source routing list of
    hosts and a source mailbox.  The first host in the <reverse-path>
    SHOULD be the host sending the MAIL command.  Similarly, the
    <forward-path> may be a source routing lists of hosts and a
    destination mailbox.  However, in general, the <forward-path> SHOULD
    contain only a mailbox and domain name, relying on the domain name
    system to supply routing information if required.  The use of source
    routes is deprecated; while servers MUST be prepared to receive and
    handle them as discussed in section 3.3 and F.2, clients SHOULD NOT
    transmit them and this section was included only to provide context.
 
    For relay purposes, the forward-path may be a source route of the
    form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE fully-
    qualified domain names.  This form is used to emphasize the
    distinction between an address and a route.  The mailbox is an
    absolute address, and the route is information about how to get
    there.  The two concepts should not be confused.
 
    If source routes are used, RFC 821 and the text below should be
    consulted for the mechanisms for constructing and updating the
    forward- and reverse-paths.
 
 
 
 
 
 
 
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    The SMTP server transforms the command arguments by moving its own
    identifier (its domain name or that of any domain for which it is
    acting as a mail exchanger), if it appears, from the forward-path to
    the beginning of the reverse-path.
 
    Notice that the forward-path and reverse-path appear in the SMTP
    commands and replies, but not necessarily in the message.  That is,
    there is no need for these paths and especially this syntax to appear
    in the "To:" , "From:", "CC:", etc. fields of the message header.
    Conversely, SMTP servers MUST NOT derive final message delivery
    information from message header fields.
 
    When the list of hosts is present, it is a "reverse" source route and
    indicates that the mail was relayed through each host on the list
    (the first host in the list was the most recent relay).  This list is
    used as a source route to return non-delivery notices to the sender.
    As each relay host adds itself to the beginning of the list, it MUST
    use its name as known in the transport environment to which it is
    relaying the mail rather than that of the transport environment from
    which the mail came (if they are different).
 
 D. Scenarios
 
    This section presents complete scenarios of several types of SMTP
    sessions.  In the examples, "C:" indicates what is said by the SMTP
    client, and "S:" indicates what is said by the SMTP server.
 
 D.1 A Typical SMTP Transaction Scenario
 
    This SMTP example shows mail sent by Smith at host bar.com, to Jones,
    Green, and Brown at host foo.com.  Here we assume that host bar.com
    contacts host foo.com directly.  The mail is accepted for Jones and
    Brown.  Green does not have a mailbox at host foo.com.
 
       S: 220 foo.com Simple Mail Transfer Service Ready
       C: EHLO bar.com
       S: 250-foo.com greets bar.com
       S: 250-8BITMIME
       S: 250-SIZE
       S: 250-DSN
       S: 250 HELP
       C: MAIL FROM:<Smith@bar.com>
       S: 250 OK
       C: RCPT TO:<Jones@foo.com>
       S: 250 OK
       C: RCPT TO:<Green@foo.com>
       S: 550 No such user here
       C: RCPT TO:<Brown@foo.com>
 
 
 
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       S: 250 OK
       C: DATA
       S: 354 Start mail input; end with <CRLF>.<CRLF>
       C: Blah blah blah...
       C: ...etc. etc. etc.
       C: .
       S: 250 OK
       C: QUIT
       S: 221 foo.com Service closing transmission channel
 
 D.2 Aborted SMTP Transaction Scenario
 
       S: 220 foo.com Simple Mail Transfer Service Ready
       C: EHLO bar.com
       S: 250-foo.com greets bar.com
       S: 250-8BITMIME
       S: 250-SIZE
       S: 250-DSN
       S: 250 HELP
       C: MAIL FROM:<Smith@bar.com>
       S: 250 OK
       C: RCPT TO:<Jones@foo.com>
       S: 250 OK
       C: RCPT TO:<Green@foo.com>
       S: 550 No such user here
       C: RSET
       S: 250 OK
       C: QUIT
       S: 221 foo.com Service closing transmission channel
 
 D.3 Relayed Mail Scenario
 
    Step 1  --  Source Host to Relay Host
 
       S: 220 foo.com Simple Mail Transfer Service Ready
       C: EHLO bar.com
       S: 250-foo.com greets bar.com
       S: 250-8BITMIME
       S: 250-SIZE
       S: 250-DSN
       S: 250 HELP
       C: MAIL FROM:<JQP@bar.com>
       S: 250 OK
       C: RCPT TO:<@foo.com:Jones@XYZ.COM>
       S: 250 OK
       C: DATA
       S: 354 Start mail input; end with <CRLF>.<CRLF>
       C: Date: Thu, 21 May 1998 05:33:29 -0700
 
 
 
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       C: From: John Q. Public <JQP@bar.com>
       C: Subject:  The Next Meeting of the Board
       C: To: Jones@xyz.com
       C:
       C: Bill:
       C: The next meeting of the board of directors will be
       C: on Tuesday.
       C:                         John.
       C: .
       S: 250 OK
       C: QUIT
       S: 221 foo.com Service closing transmission channel
 
    Step 2  --  Relay Host to Destination Host
 
       S: 220 xyz.com Simple Mail Transfer Service Ready
       C: EHLO foo.com
       S: 250 xyz.com is on the air
       C: MAIL FROM:<@foo.com:JQP@bar.com>
       S: 250 OK
       C: RCPT TO:<Jones@XYZ.COM>
       S: 250 OK
       C: DATA
       S: 354 Start mail input; end with <CRLF>.<CRLF>
       C: Received: from bar.com by foo.com ; Thu, 21 May 1998
       C:     05:33:29 -0700
       C: Date: Thu, 21 May 1998 05:33:22 -0700
       C: From: John Q. Public <JQP@bar.com>
       C: Subject:  The Next Meeting of the Board
       C: To: Jones@xyz.com
       C:
       C: Bill:
       C: The next meeting of the board of directors will be
       C: on Tuesday.
       C:                         John.
       C: .
       S: 250 OK
       C: QUIT
       S: 221 foo.com Service closing transmission channel
 
 D.4 Verifying and Sending Scenario
 
       S: 220 foo.com Simple Mail Transfer Service Ready
       C: EHLO bar.com
       S: 250-foo.com greets bar.com
       S: 250-8BITMIME
       S: 250-SIZE
       S: 250-DSN
 
 
 
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       S: 250-VRFY
       S: 250 HELP
       C: VRFY Crispin
       S: 250 Mark Crispin <Admin.MRC@foo.com>
       C: SEND FROM:<EAK@bar.com>
       S: 250 OK
       C: RCPT TO:<Admin.MRC@foo.com>
       S: 250 OK
       C: DATA
       S: 354 Start mail input; end with <CRLF>.<CRLF>
       C: Blah blah blah...
       C: ...etc. etc. etc.
       C: .
       S: 250 OK
       C: QUIT
       S: 221 foo.com Service closing transmission channel
 
 E. Other Gateway Issues
 
    In general, gateways between the Internet and other mail systems
    SHOULD attempt to preserve any layering semantics across the
    boundaries between the two mail systems involved.  Gateway-
    translation approaches that attempt to take shortcuts by mapping,
    (such as envelope information from one system to the message headers
    or body of another) have generally proven to be inadequate in
    important ways.  Systems translating between environments that do not
    support both envelopes and headers and Internet mail must be written
    with the understanding that some information loss is almost
    inevitable.
 
 F. Deprecated Features of RFC 821
 
    A few features of RFC 821 have proven to be problematic and SHOULD
    NOT be used in Internet mail.
 
 F.1 TURN
 
    This command, described in RFC 821, raises important security issues
    since, in the absence of strong authentication of the host requesting
    that the client and server switch roles, it can easily be used to
    divert mail from its correct destination.  Its use is deprecated;
    SMTP systems SHOULD NOT use it unless the server can authenticate the
    client.
 
 
 
 
 
 
 
 
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 F.2 Source Routing
 
    RFC 821 utilized the concept of explicit source routing to get mail
    from one host to another via a series of relays.  The requirement to
    utilize source routes in regular mail traffic was eliminated by the
    introduction of the domain name system "MX" record and the last
    significant justification for them was eliminated by the
    introduction, in RFC 1123, of a clear requirement that addresses
    following an "@" must all be fully-qualified domain names.
    Consequently, the only remaining justifications for the use of source
    routes are support for very old SMTP clients or MUAs and in mail
    system debugging.  They can, however, still be useful in the latter
    circumstance and for routing mail around serious, but temporary,
    problems such as problems with the relevant DNS records.
 
    SMTP servers MUST continue to accept source route syntax as specified
    in the main body of this document and in RFC 1123.  They MAY, if
    necessary, ignore the routes and utilize only the target domain in
    the address.  If they do utilize the source route, the message MUST
    be sent to the first domain shown in the address.  In particular, a
    server MUST NOT guess at shortcuts within the source route.
 
    Clients SHOULD NOT utilize explicit source routing except under
    unusual circumstances, such as debugging or potentially relaying
    around firewall or mail system configuration errors.
 
 F.3 HELO
 
    As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to
    HELO when the server will accept the former.  Servers must continue
    to accept and process HELO in order to support older clients.
 
 F.4 #-literals
 
    RFC 821 provided for specifying an Internet address as a decimal
    integer host number prefixed by a pound sign, "#".  In practice, that
    form has been obsolete since the introduction of TCP/IP.  It is
    deprecated and MUST NOT be used.
 
 F.5 Dates and Years
 
    When dates are inserted into messages by SMTP clients or servers
    (e.g., in trace fields), four-digit years MUST BE used.  Two-digit
    years are deprecated; three-digit years were never permitted in the
    Internet mail system.
 
 
 
 
 
 
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 F.6 Sending versus Mailing
 
    In addition to specifying a mechanism for delivering messages to
    user's mailboxes, RFC 821 provided additional, optional, commands to
    deliver messages directly to the user's terminal screen.  These
    commands (SEND, SAML, SOML) were rarely implemented, and changes in
    workstation technology and the introduction of other protocols may
    have rendered them obsolete even where they are implemented.
 
    Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers
    MAY implement them.  If they are implemented by servers, the
    implementation model specified in RFC 821 MUST be used and the
    command names MUST be published in the response to the EHLO command.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 Full Copyright Statement
 
    Copyright (C) The Internet Society (2001).  All Rights Reserved.
 
    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it
    or assist in its implementation may be prepared, copied, published
    and distributed, in whole or in part, without restriction of any
    kind, provided that the above copyright notice and this paragraph are
    included on all such copies and derivative works.  However, this
    document itself may not be modified in any way, such as by removing
    the copyright notice or references to the Internet Society or other
    Internet organizations, except as needed for the purpose of
    developing Internet standards in which case the procedures for
    copyrights defined in the Internet Standards process must be
    followed, or as required to translate it into languages other than
    English.
 
    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.
 
    This document and the information contained herein is provided on an
    "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
    TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
    BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
    MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 
 Acknowledgement
 
    Funding for the RFC Editor function is currently provided by the
    Internet Society.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Klensin                     Standards Track                    [Page 79]