This document 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” (STD 1) for the standardization state and status of this protocol.
If unencoded text in an 'encoded-word' contains a sequence which causes the charset interpreter to switch out of ASCII mode, it MUST contain additional control codes such that ASCII mode is again selected at the end of the 'encoded-word'.
(This rule applies separately to each 'encoded-word', including adjacent 'encoded-word's within a single header field.)¶When there is a possibility of using more than one character set to represent the text in an 'encoded-word', and in the absence of private agreements between sender and recipients of a message, it is recommended that members of the ISO-8859-* series be used in preference to other character sets.¶Initially, the legal values for "encoding" are "Q" and "B". The "Q" encoding is recommended for use when most of the characters to be encoded are in the ASCII character set; otherwise, the "B" encoding should be used.
Because RFC 822 said so little about message bodies, these documents are largely orthogonal to (rather than a revision of) RFC 822.¶RFC 2045 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 US-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 2045, 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 the most recent episode, a mime named Tape Face graced the stage.
He was dressed like a normal guy, but with tape on his mouth.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. 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.The syntax of encoded-words is such that they are unlikely to "accidentally" appear as normal text in message headers. 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 RFC 822 and RFC 2045.Finally, certain other characters are disallowed in some contexts, to ensure reliability for messages that pass through internetwork mail gateways.¶The "B" encoding automatically meets these requirements. Use of 'encoded-word's to represent strings of purely ASCII characters is allowed, but discouraged.The "Q" encoding allows a wide range of printable characters to be used in non-critical locations in the message header (e.g., Subject), with fewer characters available for use in other locations.¶The "Q" encoding is similar to the "Quoted-Printable" content-transfer-encoding defined in RFC 2045. In rare cases it may be necessary to encode ordinary text that looks like an 'encoded-word'.¶NOTE: Decoding and display of encoded-words occurs *after* a structured field body is parsed into tokens.Nevertheless, a mail reader which claims to recognize 'encoded-word's MUST be able to accept either encoding for any character set which it supports.¶Only a subset of the printable ASCII characters may be used in 'encoded-text'.