E-mail sem suporte a MIME (que é opcional) é definido pelo RFC 5322 e permite apenas caracteres ASCII "imprimíveis". O MIME estende esse padrão e permite outros esquemas de codificação para determinados campos (assunto incluído).
Ao codificar um cabeçalho, use o método "Encoded-Word" :
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.