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  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 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.
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, this document uses the term "ASCII", both for brevity and for consistency with STD 11, RFC 822. However, implementors are warned that the character set name must be spelled "US-ASCII" in MIME message and body part headers.