Simple Mail Transfer Protocol
RFC 2821

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.

Klensin                     Standards Track                     [Page 1]
RFC 2821             Simple Mail Transfer Protocol            April 2001

   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