MIME (Multipurpose Internet Mail Extensions) Part Two: Message Header Extensions for Non-ASCII Text
RFC 1522

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Network Working Group                                           K. Moore
Request for Comments: 1522                       University of Tennessee
Obsoletes: 1342                                           September 1993
Category: Standards Track

         MIME (Multipurpose Internet Mail Extensions) Part Two:
              Message Header Extensions for Non-ASCII Text

Status of this Memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo describes an extension to the message format defined in RFC
   1521 [1], to allow the representation of character sets other than
   ASCII in RFC 822 (STD 11) message headers.  The extensions described
   were designed to be highly compatible with existing Internet mail
   handling software, and to be easily implemented in mail readers that
   support RFC 1521.

1. Introduction

   RFC 1521 describes a mechanism for denoting textual body parts which
   are coded in various character sets, as well as methods for encoding
   such body parts as sequences of printable ASCII characters.  This
   memo describes similar techniques to allow the encoding of non-ASCII
   text in various portions of a RFC 822 [2] message header, in a manner
   which is unlikely to confuse existing message handling software.

   Like the encoding techniques described in RFC 1521, the techniques
   outlined here were designed to allow the use of non-ASCII characters
   in message headers in a way which is unlikely to be disturbed by the
   quirks of existing Internet mail handling programs.  In particular,
   some mail relaying programs are known to (a) delete some message
   header fields while retaining others, (b) rearrange the order of
   addresses in To or Cc fields, (c) rearrange the (vertical) order of
   header fields, and/or (d) "wrap" message headers at different places
   than those in the original message.  In addition, some mail reading
   programs are known to have difficulty correctly parsing message
   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

Moore                                                           [Page 1]
RFC 1522                     MIME Part Two                September 1993

   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.

   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 STD 11, RFC
      822 and RFC 1521.  In particular, the syntax for the ABNF used in
      this memo is defined in STD 11, RFC 822, as well as many of the
      terms used in the grammar for the header extensions defined here.
      Successful implementation of this protocol extension requires
      careful attention to the details of both STD 11, RFC 822 and RFC
      1521.

      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,