SMTP Service Extensions (RFC1869)
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
J. Klensin: AUTHOR [+4]
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. [STANDARDS-TRACK]
Network Working Group J. Klensin, WG Chair Request For Comments: 1869 MCI STD: 10 N. Freed, Editor Obsoletes: 1651 Innosoft International, Inc. Category: Standards Track M. Rose Dover Beach Consulting, Inc. E. Stefferud Network Management Associates, Inc. D. Crocker Brandenburg Consulting November 1995
SMTP Service Extensions
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.
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. Extensions to the SMTP service are registered with the 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.
The Simple Mail Transfer Protocol (SMTP)  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, et al Standards Track [Page 1]
RFC 1869 SMTP Service Extensions November 1995
(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 , whilst the body, if structured, is defined according to MIME . 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  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...