In fact it is not unreasonable to have each bcc recipient get a copy
of
the message with a bcc header that contains (only) their name in it.
In that case, it would be reasonable for an MTA to remove a Bcc header
that did not agree with the envelope address. Whether that would be
expressed as a MAY, SHOULD or MUST is another matter of course :-( .
Actually it's occasionally quite useful to be able to let two Bcc:
recipients know that they've both been sent the email. It's just that
the typical MUA is quite inflexible in letting the user determine
exactly which Bcc: recipients get to see which Bcc addresses.
Keith