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RFC 2821

Simple Mail Transfer Protocol

Pages: 79
Obsoletes:  082109741869
Obsoleted by:  5321
Updated by:  5336
Part 1 of 4 – Pages 1 to 15
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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
ToP   noToC   RFC2821 - Page 15
   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.



(page 15 continued on part 2)

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