Since RFC 2822, conforming message header names and values should be ASCII characters; values that contain non-ASCII data should use the MIME encoded-word syntax (RFC 2047) instead of a literal string. This syntax uses a string of ASCII characters indicating both the original character encoding (the "charset") and the content-transfer-encoding used to map the bytes of the charset into ASCII characters.
The form is: "=?charset?encoding?encoded text?=".
- charset may be any character set registered with IANA. Typically it would be the same charset as the message body.
- encoding can be either "
Q" denoting Q-encoding that is similar to the quoted-printable encoding, or "B" denoting base64 encoding. - encoded text is the Q-encoded or base64-encoded text.
- An encoded-word may not be more than 75 characters long, including charset, encoding, encoded text, and delimiters. If it is desirable to encode more text than will fit in an encoded-word of 75 characters, multiple encoded-words (separated by CRLF SPACE) may be used.
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