MIME (Multipurpose Internet Mail Extensions) Part Two: Message Header Extensions for Non-ASCII Text (RFC1522)
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2019-Feb-13
Internet Society Requests For Comment (RFCs)
This memo describes an extension to the message format defined in RFC 1521, 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.
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.
This memo describes an extension to the message format defined in RFC 1521 , 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.
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
Moore [Page 1]
RFC 1522 MIME Part Two September 1993
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.