I'd like to see a convention established for interpreting the
message/rfc822 type in this way, possibly when accompanied by some
other syntax.
I agree that this would be useful functionality -- I've suggested adding
it several times in the past, but was never able to get much support
for it.
Its clear that this indicator has to be on the "inside", since you want the
signature to be able to cover it. This then begs the question of whether
it should be an attribute of the signature/encryption facility or of the
MIME message/rfc822 content.
I personally favor a message/rfc822 parameter, but I can also see a case for
putting it elsewhere. What do other people think? If there seems to be
consensus that this needs to be on message/rfc822, I'd be happy to write
a short draft defining such a parameter.
Ned