Forgive me if I sound a bit niave, but wouldn't it be enough to say the
following?
For mail, the only transfer encodings that are recommended for
general use are "7bit", "quoted-printable", and "base64". For news,
"8bit" may be added to this set. Other transfer encodings may
be applicable in restricted environments. Gateways between mail
and news should take care to translate "8bit" into "quoted-printable".
It is considered an error to transmit text taken solely from the
US-ASCII subset of a ISO-8859-X character set as "8bit".
Cheers,
Rhys.