OK, this makes more sense now. Because a single message can only use one
encryption algorithm, I believe that there is no protocol question.
However, we can give implementors guidance. How does the following sound?
If a sending agent is composing a signed message to a group of recipients
where the encrypting capabilities of some of the recipients are different,
the sending agent is forced to send more than one message. It should be
noted that if the sending agent chooses to send a message encrypted with a
strong algorithm, and then send the same message encrypted with a weak
algorithm, someone watching the communications channel can decipher the
contents of the strongly-encrypted message simply by decrypting the
weakly-encrypted message.
--Paul E. Hoffman, Director
--Internet Mail Consortium