Simple Mail Transfer Protocol
draft-klensin-rfc5321bis-00

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Network Working Group                                         J. Klensin
Internet-Draft                                          December 2, 2019
Obsoletes: 5321, 1846, 7504 (if
           approved)
Updates: 1123 (if approved)
Intended status: Standards Track
Expires: June 4, 2020

                     Simple Mail Transfer Protocol
                      draft-klensin-rfc5321bis-00

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.

Note on Reading This Working Draft

   This working draft contains a relatively complete trace of
   significant changes since RFC 2821.  CREF comments marked "[2821]" or
   "[2821bis]" are pre-RFC 5321 and can be safely ignored unless there
   is some reason to reopen the related issues.  Those notes on 2821
   will be removed (and this note modified) in the -01 draft, which will
   be posted with a 3 December date.  It otherwise not expected to
   differ from this one -- readers should take their pick.  Anything
   marked "[5321bis]" is current.  In general, unless those are marked
   with "[[Note in Draft", in the contents of an "Editor's note", or are
   in the "Errata Summary" appendix, they are just notes on changes that
   have already been made and where the changes originated.  Comments
   identified as "2821ter" arose after the Last Call on what became
   RFC5321, sometimes before AUTH48 on that document or a bit later.
   Those, of course, should still be reviewed.  Surviving comments about
   rfc5321bis-00 followed by a letter indicate intermediate working
   versions of this draft and can be ignored unless the origin of
   changes is important.  As one can tell from the dates (when they are
   given), this document has been periodically updated over a very long
   period of time.

Klensin                   Expires June 4, 2020                  [Page 1]
Internet-Draft                    SMTP                     December 2019

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 4, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  . . . . . . . . . . . . . . . . . .   7
   2.  The SMTP Model  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Basic Structure . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  The Extension Model . . . . . . . . . . . . . . . . . . .  10
       2.2.1.  Background  . . . . . . . . . . . . . . . . . . . . .  10
       2.2.2.  Definition and Registration of Extensions . . . . . .  11
       2.2.3.  Special Issues with Extensions  . . . . . . . . . . .  12
     2.3.  SMTP Terminology  . . . . . . . . . . . . . . . . . . . .  12
       2.3.1.  Mail Objects  . . . . . . . . . . . . . . . . . . . .  12
       2.3.2.  Senders and Receivers . . . . . . . . . . . . . . . .  13
       2.3.3.  Mail Agents and Message Stores  . . . . . . . . . . .  13