An encoded-word may appear in a message header or body part header according to the following rules:
Ordinary ASCII text and encoded-words may appear together in the same header field. However, an encoded-word that appears in a header field defined as "*text" MUST be separated from any adjacent encoded-word or "text" by linear-white-space.
comment = "(" *(ctext / quoted-pair / comment / encoded-word) ")"
A "Q"-encoded encoded-word which appears in a comment MUST NOT contain the characters "(", ")" or " encoded-word that appears in a "comment" MUST be separated from any adjacent encoded-word or "ctext" by linear-white-space.
phrase = 1*(encoded-word / word)
In this case the set of characters that may be used in a "Q"- encoded encoded-word is restricted to: <upper and lower case ASCII letters, decimal digits, "!", "*", "+", "-", "/", "=", and "_" (underscore, ASCII 95.)>. An encoded-word that appears within a "phrase" MUST be separated from any adjacent "word", "text" or "special" by linear-white-space.
These are the ONLY locations where an encoded-word may appear. In particular, an encoded-word MUST NOT appear in any portion of an "addr-spec". In addition, an encoded-word MUST NOT be used in a Received header field.
Each encoded-word MUST encode an integral number of octets. The encoded-text in each encoded-word must be well-formed according to the encoding specified; the encoded-text may not be continued in the next encoded-word. (For example, "=?charset?Q?=?= =?charset?Q?AB?=" would be illegal, because the two hex digits "AB" must follow the "=" in the same encoded-word.)
Each encoded-word MUST represent an integral number of characters. A multi-octet character may not be split across adjacent encoded-words.
Only printable and white space character data should be encoded using this scheme. However, since these encoding schemes allow the encoding of arbitrary octet values, mail readers that implement this decoding should also ensure that display of the decoded data on the recipient's terminal will not cause unwanted side-effects.
Use of these methods to encode non-textual data (e.g., pictures or sounds) is not defined by this memo. Use of encoded-words to represent strings of purely ASCII characters is allowed, but discouraged. In rare cases it may be necessary to encode ordinary text that looks like an encoded-word.