On Apr 15, 2012, at 3:56 PM, John C Klensin wrote:
If I correctly understand the question, I simply put a "bcc"
field in the message as I submit it to the MUA. It removes that
field. constructs the "blind copy" warning text, and constructs
the envelope(s) appropriately (typically one envelope for the
set of To and Cc recipients and one for each Bcc one) for
handoff to the Submission server. 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.
A submission server will have some difficulty adding a
warning message that'll be visible to a typical recipient
(multipart/mime, non plain-text, DKIM, PGP, S/MIME…).
Handling Bcc by copying the Bcc header to the envelope,
then deleting the header is fine, though I wouldn't trust any
MUA that relied on that functionality.
Cheers,
Steve