Tech-invite3GPPspecsSIPRFCs
898887868584838281807978777675747372717069686766656463626160595857565554535251504948474645444342414039383736353433323130292827262524232221201918171615141312111009080706050403020100

in Index   Prev   Next

RFC 5321

Simple Mail Transfer Protocol

Pages: 95
Draft Standard
Errata
Obsoletes:  2821
Updates:  1123
Updated by:  7504
Part 1 of 4 – Pages 1 to 17
None   None   Next

Top   ToC   RFC5321 - Page 1
Network Working Group                                         J. Klensin
Request for Comments: 5321                                  October 2008
Obsoletes: 2821
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.

Abstract

This document is a specification of the basic protocol for Internet electronic mail transport. It consolidates, updates, and clarifies several previous documents, making all or parts of most of them obsolete. It covers the SMTP extension mechanisms and best practices for the contemporary Internet, but does not provide details about particular extensions. 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 for "split-UA" (User Agent) mail reading systems and mobile environments.
Top   ToC   RFC5321 - Page 2

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Transport of Electronic Mail . . . . . . . . . . . . . . . 5 1.2. History and Context for This Document . . . . . . . . . . 5 1.3. Document Conventions . . . . . . . . . . . . . . . . . . . 6 2. The SMTP Model . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Basic Structure . . . . . . . . . . . . . . . . . . . . . 7 2.2. The Extension Model . . . . . . . . . . . . . . . . . . . 9 2.2.1. Background . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Definition and Registration of Extensions . . . . . . 10 2.2.3. Special Issues with Extensions . . . . . . . . . . . . 11 2.3. SMTP Terminology . . . . . . . . . . . . . . . . . . . . . 11 2.3.1. Mail Objects . . . . . . . . . . . . . . . . . . . . . 11 2.3.2. Senders and Receivers . . . . . . . . . . . . . . . . 12 2.3.3. Mail Agents and Message Stores . . . . . . . . . . . . 12 2.3.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.5. Domain Names . . . . . . . . . . . . . . . . . . . . . 13 2.3.6. Buffer and State Table . . . . . . . . . . . . . . . . 14 2.3.7. Commands and Replies . . . . . . . . . . . . . . . . . 14 2.3.8. Lines . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.9. Message Content and Mail Data . . . . . . . . . . . . 15 2.3.10. Originator, Delivery, Relay, and Gateway Systems . . . 15 2.3.11. Mailbox and Address . . . . . . . . . . . . . . . . . 15 2.4. General Syntax Principles and Transaction Model . . . . . 16 3. The SMTP Procedures: An Overview . . . . . . . . . . . . . . . 17 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . . 18 3.2. Client Initiation . . . . . . . . . . . . . . . . . . . . 18 3.3. Mail Transactions . . . . . . . . . . . . . . . . . . . . 19 3.4. Forwarding for Address Correction or Updating . . . . . . 21 3.5. Commands for Debugging Addresses . . . . . . . . . . . . . 22 3.5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 22 3.5.2. VRFY Normal Response . . . . . . . . . . . . . . . . . 24 3.5.3. Meaning of VRFY or EXPN Success Response . . . . . . . 25 3.5.4. Semantics and Applications of EXPN . . . . . . . . . . 26 3.6. Relaying and Mail Routing . . . . . . . . . . . . . . . . 26 3.6.1. Source Routes and Relaying . . . . . . . . . . . . . . 26 3.6.2. Mail eXchange Records and Relaying . . . . . . . . . . 26 3.6.3. Message Submission Servers as Relays . . . . . . . . . 27 3.7. Mail Gatewaying . . . . . . . . . . . . . . . . . . . . . 28 3.7.1. Header Fields in Gatewaying . . . . . . . . . . . . . 28 3.7.2. Received Lines in Gatewaying . . . . . . . . . . . . . 29 3.7.3. Addresses in Gatewaying . . . . . . . . . . . . . . . 29 3.7.4. Other Header Fields in Gatewaying . . . . . . . . . . 29 3.7.5. Envelopes in Gatewaying . . . . . . . . . . . . . . . 30 3.8. Terminating Sessions and Connections . . . . . . . . . . . 30 3.9. Mailing Lists and Aliases . . . . . . . . . . . . . . . . 31 3.9.1. Alias . . . . . . . . . . . . . . . . . . . . . . . . 31
Top   ToC   RFC5321 - Page 3
       3.9.2.  List . . . . . . . . . . . . . . . . . . . . . . . . . 31
   4.  The SMTP Specifications  . . . . . . . . . . . . . . . . . . . 32
     4.1.  SMTP Commands  . . . . . . . . . . . . . . . . . . . . . . 32
       4.1.1.  Command Semantics and Syntax . . . . . . . . . . . . . 32
       4.1.2.  Command Argument Syntax  . . . . . . . . . . . . . . . 41
       4.1.3.  Address Literals . . . . . . . . . . . . . . . . . . . 43
       4.1.4.  Order of Commands  . . . . . . . . . . . . . . . . . . 44
       4.1.5.  Private-Use Commands . . . . . . . . . . . . . . . . . 46
     4.2.  SMTP Replies . . . . . . . . . . . . . . . . . . . . . . . 46
       4.2.1.  Reply Code Severities and Theory . . . . . . . . . . . 48
       4.2.2.  Reply Codes by Function Groups . . . . . . . . . . . . 50
       4.2.3.  Reply Codes in Numeric Order . . . . . . . . . . . . . 52
       4.2.4.  Reply Code 502 . . . . . . . . . . . . . . . . . . . . 53
       4.2.5.  Reply Codes after DATA and the Subsequent
               <CRLF>.<CRLF>  . . . . . . . . . . . . . . . . . . . . 53
     4.3.  Sequencing of Commands and Replies . . . . . . . . . . . . 54
       4.3.1.  Sequencing Overview  . . . . . . . . . . . . . . . . . 54
       4.3.2.  Command-Reply Sequences  . . . . . . . . . . . . . . . 55
     4.4.  Trace Information  . . . . . . . . . . . . . . . . . . . . 57
     4.5.  Additional Implementation Issues . . . . . . . . . . . . . 61
       4.5.1.  Minimum Implementation . . . . . . . . . . . . . . . . 61
       4.5.2.  Transparency . . . . . . . . . . . . . . . . . . . . . 62
       4.5.3.  Sizes and Timeouts . . . . . . . . . . . . . . . . . . 62
         4.5.3.1.  Size Limits and Minimums . . . . . . . . . . . . . 62
           4.5.3.1.1.  Local-part . . . . . . . . . . . . . . . . . . 63
           4.5.3.1.2.  Domain . . . . . . . . . . . . . . . . . . . . 63
           4.5.3.1.3.  Path . . . . . . . . . . . . . . . . . . . . . 63
           4.5.3.1.4.  Command Line . . . . . . . . . . . . . . . . . 63
           4.5.3.1.5.  Reply Line . . . . . . . . . . . . . . . . . . 63
           4.5.3.1.6.  Text Line  . . . . . . . . . . . . . . . . . . 63
           4.5.3.1.7.  Message Content  . . . . . . . . . . . . . . . 63
           4.5.3.1.8.  Recipients Buffer  . . . . . . . . . . . . . . 64
           4.5.3.1.9.  Treatment When Limits Exceeded . . . . . . . . 64
           4.5.3.1.10. Too Many Recipients Code . . . . . . . . . . . 64
         4.5.3.2.  Timeouts . . . . . . . . . . . . . . . . . . . . . 65
           4.5.3.2.1.  Initial 220 Message: 5 Minutes . . . . . . . . 65
           4.5.3.2.2.  MAIL Command: 5 Minutes  . . . . . . . . . . . 65
           4.5.3.2.3.  RCPT Command: 5 Minutes  . . . . . . . . . . . 65
           4.5.3.2.4.  DATA Initiation: 2 Minutes . . . . . . . . . . 66
           4.5.3.2.5.  Data Block: 3 Minutes  . . . . . . . . . . . . 66
           4.5.3.2.6.  DATA Termination: 10 Minutes.  . . . . . . . . 66
           4.5.3.2.7.  Server Timeout: 5 Minutes. . . . . . . . . . . 66
       4.5.4.  Retry Strategies . . . . . . . . . . . . . . . . . . . 66
       4.5.5.  Messages with a Null Reverse-Path  . . . . . . . . . . 68
   5.  Address Resolution and Mail Handling . . . . . . . . . . . . . 69
     5.1.  Locating the Target Host . . . . . . . . . . . . . . . . . 69
     5.2.  IPv6 and MX Records  . . . . . . . . . . . . . . . . . . . 71
   6.  Problem Detection and Handling . . . . . . . . . . . . . . . . 71
Top   ToC   RFC5321 - Page 4
     6.1.  Reliable Delivery and Replies by Email . . . . . . . . . . 71
     6.2.  Unwanted, Unsolicited, and "Attack" Messages . . . . . . . 72
     6.3.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 73
     6.4.  Compensating for Irregularities  . . . . . . . . . . . . . 73
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 75
     7.1.  Mail Security and Spoofing . . . . . . . . . . . . . . . . 75
     7.2.  "Blind" Copies . . . . . . . . . . . . . . . . . . . . . . 76
     7.3.  VRFY, EXPN, and Security . . . . . . . . . . . . . . . . . 76
     7.4.  Mail Rerouting Based on the 251 and 551 Response Codes . . 77
     7.5.  Information Disclosure in Announcements  . . . . . . . . . 77
     7.6.  Information Disclosure in Trace Fields . . . . . . . . . . 78
     7.7.  Information Disclosure in Message Forwarding . . . . . . . 78
     7.8.  Resistance to Attacks  . . . . . . . . . . . . . . . . . . 78
     7.9.  Scope of Operation of SMTP Servers . . . . . . . . . . . . 78
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 79
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 80
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 81
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 81
     10.2. Informative References . . . . . . . . . . . . . . . . . . 82
   Appendix A.  TCP Transport Service . . . . . . . . . . . . . . . . 85
   Appendix B.  Generating SMTP Commands from RFC 822 Header
                Fields  . . . . . . . . . . . . . . . . . . . . . . . 85
   Appendix C.  Source Routes . . . . . . . . . . . . . . . . . . . . 86
   Appendix D.  Scenarios . . . . . . . . . . . . . . . . . . . . . . 87
     D.1.  A Typical SMTP Transaction Scenario  . . . . . . . . . . . 88
     D.2.  Aborted SMTP Transaction Scenario  . . . . . . . . . . . . 89
     D.3.  Relayed Mail Scenario  . . . . . . . . . . . . . . . . . . 90
     D.4.  Verifying and Sending Scenario . . . . . . . . . . . . . . 92
   Appendix E.  Other Gateway Issues  . . . . . . . . . . . . . . . . 92
   Appendix F.  Deprecated Features of RFC 821  . . . . . . . . . . . 93
     F.1.  TURN . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
     F.2.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 93
     F.3.  HELO . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
     F.4.  #-literals . . . . . . . . . . . . . . . . . . . . . . . . 94
     F.5.  Dates and Years  . . . . . . . . . . . . . . . . . . . . . 94
     F.6.  Sending versus Mailing . . . . . . . . . . . . . . . . . . 94
Top   ToC   RFC5321 - Page 5

1. Introduction

1.1. Transport of Electronic Mail

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 [1] describe some of them. An important feature of SMTP is its capability to transport mail across multiple networks, usually referred to as "SMTP mail relaying" (see Section 3.6). 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 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 (RFC 1035 [2], RFC 974 [12], and Section 5 of this document) are used to identify the appropriate next-hop destination for a message being transported.

1.2. History and Context for This Document

This document is a specification of the basic protocol for the Internet electronic mail transport. It consolidates, updates and clarifies, but does not add new or change existing functionality of the following: o the original SMTP (Simple Mail Transfer Protocol) specification of RFC 821 [1], o domain name system requirements and implications for mail transport from RFC 1035 [2] and RFC 974 [12], o the clarifications and applicability statements in RFC 1123 [3], and o material drawn from the SMTP Extension mechanisms in RFC 1869 [13].
Top   ToC   RFC5321 - Page 6
   o  Editorial and clarification changes to RFC 2821 [14] to bring that
      specification to Draft Standard.

   It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updates RFC
   1123 (replacing 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.

   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 Post Office
   Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
   [17]).  In general, the separate mail submission protocol specified
   in RFC 4409 [18] is now preferred to direct use of SMTP; more
   discussion of that subject appears in that document.

   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, RFC 5322 [4], discusses message header sections
   and bodies and specifies formats and structures for them.

1.3. Document Conventions

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 in RFC 2119 [5]. As each of these terms was intentionally and carefully chosen to improve the interoperability of email, each use of these terms is to be treated as a conformance requirement. Because this document has a long history and to avoid the risk of various errors and of confusing readers and documents that point to this one, most examples and the domain names they contain are preserved from RFC 2821. Readers are cautioned that these are
Top   ToC   RFC5321 - Page 7
   illustrative examples that should not actually be used in either code
   or configuration files.

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 identifier(s) ("names") of the domain(s) to which mail messages are to be transferred, is a local matter, and is not addressed by this document. In some cases, the designated domain(s), or those 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 (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17]) protocols, or when the SMTP client is inside an isolated transport service environment, the domain 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 domains 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 ones, and their destinations, are expected to support all of the queuing, retrying, and alternate address functions discussed in this specification. In many situations and configurations, the less- capable clients discussed above SHOULD be using the message submission protocol (RFC 4409 [18]) rather than SMTP.
Top   ToC   RFC5321 - Page 8
   The means by which an SMTP client, once it has determined a target
   domain, 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, once the server has issued a success response at the end of the
   mail data, a formal handoff of responsibility for the message occurs:
   the protocol requires that a server MUST accept responsibility for
   either delivering the message or properly reporting the failure to do
   so (see Sections 6.1, 6.2, and 7.8, below).

   Once the transmission channel is established and initial handshaking
   is 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 lines in the header section 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 upon extension
   requests such as command pipelining (RFC 2920 [19]).

   Once a given mail message has been transmitted, the client may either
   request that the connection be shut down or may initiate other mail
Top   ToC   RFC5321 - Page 9
   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.  Historically, this transmission normally
   occurred 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.  A very
   common case in the Internet today involves submission of the original
   message to an intermediate, "message submission" server, which is
   similar to a relay but has some additional properties; such servers
   are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
   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 Appendix C and
   Appendix 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
Top   ToC   RFC5321 - Page 10
   added, it must be done in a way that permits older implementations to
   continue working acceptably.  The extension framework consists of:

   o  The SMTP command EHLO, superseding the earlier HELO,

   o  a registry of SMTP service extensions,

   o  additional parameters to the SMTP MAIL and RCPT commands, and

   o  optional replacements for commands defined in this protocol, such
      as for DATA in non-ASCII transmissions (RFC 3030 [20]).

   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: o the textual name of the SMTP service extension; o the EHLO keyword value associated with the extension; o the syntax and possible values of parameters associated with the EHLO keyword value; o 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); o any new parameters the extension associates with the MAIL or RCPT verbs; o a description of how support for the extension affects the behavior of a server and client SMTP; and
Top   ToC   RFC5321 - Page 11
   o  the increment by which the extension is increasing the maximum
      length of the commands MAIL and/or RCPT, over that specified in
      this Standard.

   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.

2.2.3. Special Issues with Extensions

Extensions that change fairly basic properties of SMTP operation are permitted. The text in other sections of this document must be understood in that context. In particular, extensions can change the minimum limits specified in Section 4.5.3, can change the ASCII character set requirement as mentioned above, or can introduce some optional modes of message handling. In particular, if an extension implies that the delivery path normally supports special features of that extension, and an intermediate SMTP system finds a next hop that does not support the required extension, it MAY choose, based on the specific extension and circumstances, to requeue the message and try later and/or try an alternate MX host. If this strategy is employed, the timeout to fall back to an unextended format (if one is available) SHOULD be less than the normal timeout for bouncing as undeliverable (e.g., if normal timeout is three days, the requeue timeout before attempting to transmit the mail without the extension might be one day).

2.3. SMTP Terminology

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
Top   ToC   RFC5321 - Page 12
   which error reports should be directed), one or more recipient
   addresses, and optional protocol extension material.  Historically,
   variations on the reverse-path (originator) address specification
   command (MAIL) could be used to specify alternate delivery modes,
   such as immediate display; those variations have now been deprecated
   (see Appendix F and Appendix F.6).

   The SMTP content is sent in the SMTP DATA protocol unit and has two
   parts: the header section and the body.  If the content conforms to
   other contemporary standards, the header section consists of a
   collection of header fields, each consisting of a header name, a
   colon, and data, structured as in the message format specification
   (RFC 5322 [4]); the body, if structured, is defined according to MIME
   (RFC 2045 [21]).  The content is textual in nature, expressed using
   the US-ASCII repertoire [6].  Although SMTP extensions (such as
   "8BITMIME", RFC 1652 [22]) may relax this restriction for the content
   body, the content header fields are always encoded using the US-ASCII
   repertoire.  Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
   define 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 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
Top   ToC   RFC5321 - Page 13
   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 the next section); they SHOULD NOT be identified by numerical addresses, i.e., by address literals as described in Section 4.1.2.

2.3.5. Domain Names

A domain name (or often just a "domain") consists of one or more components, separated by dots if more than one appears. In the case of a top-level domain used by itself in an email address, a single string is used without any dots. This makes the requirement, described in more detail below, that only fully-qualified domain names appear in SMTP transactions on the public Internet, particularly important where top-level domains are involved. These components ("labels" in DNS terminology, RFC 1035 [2]) are restricted for SMTP purposes to consist of a sequence of letters, digits, and hyphens drawn from the ASCII character set [6]. 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 RFC 1035 [2] and Section 5 of this specification. The domain name, as described in this document and in RFC 1035 [2], 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. 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 address (i.e., A or AAAA) RRs (as discussed in Section 5) are permitted, as are CNAME RRs whose targets can be resolved, in turn, to MX or address RRs. Local nicknames or unqualified names MUST NOT be used. There are two exceptions to the rule requiring FQDNs: o The domain name given in the EHLO command MUST be either a primary host name (a domain name that resolves to an address RR) or, if the host has no name, an address literal, as described in Section 4.1.3 and discussed further in the EHLO discussion of Section 4.1.4.
Top   ToC   RFC5321 - Page 14
   o  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.

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 that 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.

2.3.7. Commands and Replies

SMTP commands and, unless altered by a service extension, message data, are transmitted from the sender to the receiver via the transmission channel in "lines". An SMTP reply is an acknowledgment (positive or negative) sent in "lines" 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. RFC 3463 [25], specifies further structuring of the reply strings, including the use of supplemental and more specific completion codes (see also RFC 5248 [26]).

2.3.8. 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). 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.
Top   ToC   RFC5321 - Page 15

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 the message header section and the possibly structured message body. The MIME specification (RFC 2045 [21]) provides the standard mechanisms for structured message bodies.

2.3.10. 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 that 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 RFC 2979 [27]).

2.3.11. 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
Top   ToC   RFC5321 - Page 16
   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.

2.4. 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 are required following 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. 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. 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. 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. Mailbox domains follow normal DNS rules and are hence not case sensitive. 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 clause 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 [6]. 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 7-bit
Top   ToC   RFC5321 - Page 17
   transport only.  An originating SMTP client that has not successfully
   negotiated an appropriate extension with a particular server (see the
   next paragraph) 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
   the ultimate delivery of a severely garbled message to the recipient.
   Delivery SMTP systems MAY reject such messages, or return them as
   undeliverable, rather than deliver them.  In the absence of a server-
   offered extension explicitly permitting it, a sending SMTP system is
   not permitted to send envelope commands in any character set other
   than US-ASCII.  Receiving systems SHOULD reject such commands,
   normally using "500 syntax error - invalid character" replies.

   8-bit message content transmission MAY be requested of the server by
   a client using extended SMTP facilities, notably the "8BITMIME"
   extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
   servers.  However, it MUST NOT be construed as authorization to
   transmit unrestricted 8-bit material, nor does 8BITMIME authorize
   transmission of any envelope material in other than ASCII. 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 in RFC 5234 [7].  Metalanguage terms used in running
   text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
   The reader is cautioned that the grammar expressed in the
   metalanguage is not comprehensive.  There are many instances in which
   provisions in the text constrain or otherwise modify the syntax or
   semantics implied by the grammar.



(page 17 continued on part 2)

Next Section