RFC 2045 Internet Message Bodies November 1996
1. Introduction
Since its publication in 1982, RFC 822 has defined the standard
format of textual mail messages on the Internet. Its success has
been such that the RFC 822 format has been adopted, wholly or
partially, well beyond the confines of the Internet and the Internet
SMTP transport defined by RFC 821. As the format has seen wider use,
a number of limitations have proven increasingly restrictive for the
user community.
RFC 822 was intended to specify a format for text messages. As such,
non-text messages, such as multimedia messages that might include
audio or images, are simply not mentioned. Even in the case of text,
however, RFC 822 is inadequate for the needs of mail users whose
languages require the use of character sets richer than US-ASCII.
Since RFC 822 does not specify mechanisms for mail containing audio,
video, Asian language text, or even text in most European languages,
additional specifications are needed.
One of the notable limitations of RFC 821/822 based mail systems is
the fact that they limit the contents of electronic mail messages to
relatively short lines (e.g. 1000 characters or less [RFC-821]) of
7bit US-ASCII. This forces users to convert any non-textual data
that they may wish to send into seven-bit bytes representable as
printable US-ASCII characters before invoking a local mail UA (User
Agent, a program with which human users send and receive mail).
Examples of such encodings currently used in the Internet include
pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in
RFC 1421, the Andrew Toolkit Representation [ATK], and many others.
The limitations of RFC 822 mail become even more apparent as gateways
are designed to allow for the exchange of mail messages between RFC
822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
inclusion of non-textual material within electronic mail messages.
The current standards for the mapping of X.400 messages to RFC 822
messages specify either that X.400 non-textual material must be
converted to (not encoded in) IA5Text format, or that they must be
discarded, notifying the RFC 822 user that discarding has occurred.
This is clearly undesirable, as information that a user may wish to
receive is lost. Even though a user agent may not have the
capability of dealing with the non-textual material, the user might
have some mechanism external to the UA that can extract useful
information from the material. Moreover, it does not allow for the
fact that the message may eventually be gatewayed back into an X.400
message handling system (i.e., the X.400 message is "tunneled"
through Internet mail), where the non-textual information would
definitely become useful again.
Freed & Borenstein Standards Track [Page 3]