RFC 1651 (rfc1651) - Page 2 of 11
SMTP Service Extensions
Alternative Format: Original Text Document
RFC 1651 SMTP Service Extensions July 1994
(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 STD 11, 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