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.

However, implementors are warned that the character set name must be spelled "US-ASCII" in MIME message and body part headers.¶This memo specifies a protocol for the representation of non-ASCII text in message headers.

It specifically DOES NOT define any translation between "8-bit headers" and pure ASCII headers, nor is any such translation assumed to be possible.¶An 'encoded-word' may not be more than 75 characters long, including 'charset', 'encoding', 'encoded-text', and delimiters.

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.

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 "=? In particular, the syntax for the ABNF used in this memo is defined in RFC 822, as well as many of the terminal or nonterminal symbols from RFC 822 are used in the grammar for the header extensions defined here.

Among the symbols defined in RFC 822 and referenced in this memo are: 'addr-spec', 'atom', 'CHAR', 'comment', 'CTLs', 'ctext', 'linear-white-space', 'phrase', 'quoted-pair'. Successful implementation of this protocol extension requires careful attention to the RFC 822 definitions of these terms.¶When the term "ASCII" appears in this memo, it refers to the "7-Bit American Standard Code for Information Interchange", ANSI X3.4-1986.

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 "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. 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.

As a consequence, unencoded white space characters (such as SPACE and HTAB) are FORBIDDEN within an 'encoded-word'.