Of course, if one had a submission server that was smart about these
things, the MUA could hand the message, including the Bcc field, to
it and let it sort out envelopes (which it needs to do anyway) and
the warning message.
That strikes me as a really unfortunate idea. Submission servers know
all about envelopes and message headers. In most cases, they don't
know anything about the body of the message beyond what they can
intuit from the MIME headers.
If your submission server adds text to a body saying something like
"you are a blind copy recipient", in what language is that message
expressed? If you'd rather it use another language, how can you tell
it to do so? This all belongs in the MUA.
While there is plenty of historical precedent for submission servers
fixing up headers, adding missing ones that can be constructed
mechanically, and creating an envelope based on recipient addresses in
the headers, I can't think of fixups they do to message bodies other
than perhaps wrapping the whole unmodified body in a crypto signature.
I'd rather move toward MUAs creating correct messages, and having
submission server fixups as a backup.
R's,
John