SMTP Service Extensions
RFC 1425

Document Type RFC - Proposed Standard (February 1993; No errata)
Obsoleted by RFC 1651
Last updated 2013-03-02
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Network Working Group                               J. Klensin, WG Chair
Request for Comments: 1425                     United Nations University
                                                        N. Freed, Editor
                                            Innosoft International, Inc.
                                                                 M. Rose
                                            Dover Beach Consulting, Inc.
                                                            E. Stefferud
                                     Network Management Associates, Inc.
                                                              D. Crocker
                                                       The Branch Office
                                                           February 1993

                        SMTP Service Extensions

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

1.  Abstract

   This memo defines a framework for extending the SMTP service by
   defining a means whereby a server SMTP can inform a client SMTP as to
   the service extensions it supports. Standard extensions to the SMTP
   service are registered with the Internet Assigned Numbers Authority
   (IANA).  This framework does not require modification of existing
   SMTP clients or servers unless the features of the service extensions
   are to be requested or provided.

2.  Introduction

   The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
   effective basis for the relay function of message transfer agents.
   Although a decade old, SMTP has proven remarkably resilient.
   Nevertheless, the need for a number of protocol extensions has become
   evident. Rather than describing these extensions as separate and
   haphazard entities, this document enhances SMTP in a straightforward
   fashion that provides a framework in which all future extensions can
   be built in a single consistent way.

3.  Framework for SMTP Extensions

   For the purpose of service extensions to SMTP, SMTP relays a mail
   object containing an envelope and a content.

Klensin, Freed, Rose, Stefferud & Crocker                       [Page 1]
RFC 1425                SMTP Service Extensions            February 1993

          (1)  The SMTP envelope is straightforward, and is sent as a
               series of SMTP protocol units: it consists of an
               originator address (to which error reports should be
               directed); a delivery mode (e.g., deliver to recipient
               mailboxes); and, one or more recipient addresses.

          (2)  The SMTP content is sent in the SMTP DATA protocol unit
               and has two parts: the headers and the body. The headers
               form a collection of field/value pairs structured
               according to RFC 822 [2], whilst the body, if structured,
               is defined according to MIME [3]. The content is textual
               in nature, expressed using the US ASCII repertoire (ANSI
               X3.4-1986). Although extensions (such as MIME) may relax
               this restriction for the content body, the content
               headers are always encoded using the US ASCII repertoire.
               The algorithm defined in [4] is used to represent header
               values outside the US ASCII repertoire, whilst still
               encoding them using the US ASCII repertoire.

   Although SMTP is widely and robustly deployed, some parts of the
   Internet community might wish to extend the SMTP service.  This memo
   defines a means whereby both an extended SMTP client and server may
   recognize each other as such and the server can inform the client as
   to the service extensions that it supports.

   It must be emphasized that any extension to the SMTP service should
   not be considered lightly. SMTP's strength comes primarily from its
   simplicity.  Experience with many protocols has shown that:

               protocols with few options tend towards ubiquity, whilst
               protocols with many options tend towards obscurity.

   This means that 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.

   Given this environment, the framework for the extensions described in
   this memo consists of:

          (1)  a new SMTP command (section 4)

          (2)  a registry of SMTP service extensions (section 5)

          (3)  additional parameters to the SMTP MAIL FROM and RCPT TO
               commands (section 6).

Klensin, Freed, Rose, Stefferud & Crocker                       [Page 2]
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