If the mail reader does not support the character set used, it may
   (a) display the encoded-word as ordinary text (i.e., as it appears in
   the header), (b) make a "best effort" to display using such
   characters as are available, or (c) substitute an appropriate message
   indicating that the decoded text could not be displayed.

   If the character set being used employs code-switching techniques,
   display of the encoded text implicitly begins in "ASCII mode".  In
   addition, the mail reader must ensure that the output device is once
   again in "ASCII mode" after the encoded-word is displayed.

6.3. Mail reader handling of incorrectly formed encoded-words

   It is possible that an encoded-word that is legal according to the
   syntax defined in section 2, is incorrectly formed according to the
   rules for the encoding being used.   For example:

   (1) An encoded-word which contains characters which are not legal for
       a particular encoding (for example, a '-' in the "B" encoding),
       is incorrectly formed.

   (2) Any encoded-word which encodes a non-integral number of
       characters or octets is incorrectly formed.

   A mail reader need not attempt to display the text associated with an
   encoded-word that is incorrectly formed.  However, a mail reader MUST
   NOT prevent the display or handling of a message because an encoded-
   word is incorrectly formed.

7. Conformance

   A mail composing program claiming compliance with this specification
   MUST ensure that any string of non-white-space printable ASCII
   characters within a "*text" or "*ctext" that begins with "=?" and
   ends with "?=" be a valid encoded-word.  ("begins" means: at the
   start of the field-body or immediately following linear-white-space;
   "ends" means: at the end of the field-body or immediately preceding
   linear-white-space.) In addition, any "word" within a "phrase" that
   begins with "=?" and ends with "?=" must be a valid encoded-word.

   A mail reading program claiming compliance with this specification
   must be able to distinguish encoded-words from "text", "ctext", or
   "word"s, according to the rules in section 6, anytime they appear in
   appropriate places in message headers.  It must support both the "B"
   and "Q" encodings for any character set which it supports.  The
   program must be able to display the unencoded text if the character




 
RFC 1522                     MIME Part Two                September 1993


   set is "US-ASCII".  For the ISO-8859-* character sets, the mail
   reading program must at least be able to display the characters which
   are also in the ASCII set.

8. Examples

      From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu>
      To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <keld@dkuug.dk>
      CC: =?ISO-8859-1?Q?Andr=E9_?= Pirard <PIRARD@vm1.ulg.ac.be>
      Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=
       =?ISO-8859-2?B?dSB1bmRlcnN0YW5kIHRoZSBleGFtcGxlLg==?=

      From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <ojarnef@admin.kth.se>
      To: ietf-822@dimacs.rutgers.edu, ojarnef@admin.kth.se
      Subject: Time for ISO 10646?

      To: Dave Crocker <dcrocker@mordor.stanford.edu>
      Cc: ietf-822@dimacs.rutgers.edu, paf@comsol.se
      From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <paf@nada.kth.se>
      Subject: Re: RFC-HDR care and feeding

      From: Nathaniel Borenstein <nsb@thumper.bellcore.com>
            (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)
      To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed
         <ned@innosoft.com>, Keith Moore <moore@cs.utk.edu>
      Subject: Test of new header generator
      MIME-Version: 1.0
      Content-type: text/plain; charset=ISO-8859-1

9. References

   [1] Borenstein N., and N. Freed, "MIME (Multipurpose Internet Mail
       Extensions) Part One:  Mechanisms for Specifying and Describing
       the Format of Internet Message Bodies", RFC 1521, Bellcore,
       Innosoft, September 1993.

   [2] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, UDEL, August 1982.

   [3] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
       RFC 1340, USC/Information Sciences Institute, July 1992.