RFC 2047 (rfc2047) - Page 3 of 15
MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text
Alternative Format: Original Text Document
RFC 2047 Message Header Extensions November 1996
Instead, certain sequences of "ordinary" printable ASCII characters
(known as "encoded-words") are reserved for use as encoded data. The
syntax of encoded-words is such that they are unlikely to
"accidentally" appear as normal text in message headers.
Furthermore, the characters used in encoded-words are restricted to
those which do not have special meanings in the context in which the
encoded-word appears.
Generally, an "encoded-word" is a sequence of printable ASCII
characters that begins with "=?", ends with "?=", and has two "?"s in
between. It specifies a character set and an encoding method, and
also includes the original text encoded as graphic ASCII characters,
according to the rules for that encoding method.
A mail composer that implements this specification will provide a
means of inputting non-ASCII text in header fields, but will
translate these fields (or appropriate portions of these fields) into
encoded-words before inserting them into the message header.
A mail reader that implements this specification will recognize
encoded-words when they appear in certain portions of the message
header. Instead of displaying the encoded-word "as is", it will
reverse the encoding and display the original text in the designated
character set.
NOTES
This memo relies heavily on notation and terms defined RFC 822 and
RFC 2045. In particular, the syntax for the ABNF used in this memo
is defined in RFC 822, as well as many of the terminal or nonterminal
symbols from RFC 822 are used in the grammar for the header
extensions defined here. Among the symbols defined in RFC 822 and
referenced in this memo are: 'addr-spec', 'atom', 'CHAR', 'comment',
'CTLs', 'ctext', 'linear-white-space', 'phrase', 'quoted-pair'.
'quoted-string', 'SPACE', and 'word'. Successful implementation of
this protocol extension requires careful attention to the RFC 822
definitions of these terms.
When the term "ASCII" appears in this memo, it refers to the "7-Bit
American Standard Code for Information Interchange", ANSI X3.4-1986.
The MIME charset name for this character set is "US-ASCII". When not
specifically referring to the MIME charset name, this document uses
the term "ASCII", both for brevity and for consistency with RFC 822.
However, implementors are warned that the character set name must be
spelled "US-ASCII" in MIME message and body part headers.
Moore Standards Track