headers which, while legal according to RFC 822, make use of
   backslash-quoting to "hide" special characters such as "<", ",", or
   ":", or which exploit other infrequently-used features of that
   specification.

   While it is unfortunate that these programs do not correctly
   interpret RFC 822 headers, to "break" these programs would cause
   severe operational problems for the Internet mail system.  The
   extensions described in this memo therefore do not rely on little-
   used features of RFC 822.




 
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.







 
RFC 2047               Message Header Extensions           November 1996


   This memo specifies a protocol for the representation of non-ASCII
   text in message headers.  It specifically DOES NOT define any
   translation between "8-bit headers" and pure ASCII headers, nor is
   any such translation assumed to be possible.

2. Syntax of encoded-words

   An 'encoded-word' is defined by the following ABNF grammar.  The
   notation of RFC 822 is used, with the exception that white space
   characters MUST NOT appear between components of an 'encoded-word'.

   encoded-word = "=?" charset "?" encoding "?" encoded-text "?="

   charset = token    ; see section 3

   encoding = token   ; see section 4

   token = 1*<Any CHAR except SPACE, CTLs, and especials>

   especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
               <"> / "/" / "[" / "]" / "?" / "." / "="

   encoded-text = 1*<Any printable ASCII character other than "?"
                     or SPACE>
                  ; (but see "Use of encoded-words in message
                  ; headers", section 5)