[Mimedefang] MDN != DSN (WAS: how to disable notify=success)

[Dirk the Daring]

   Folx, don't confuse Delivery Status Notification (RFC 1891, part of
ESMTP and implemented on MTAs) with Mail Disposition Notification (RFC
3798, implemented by MUAs, not MTAs).

   "notify=success" is DSN and takes place during the ESMTP
conversation, specifically the RCPT TO step. It does not involve a
header (altho modern versions of sendmail, specifically, can convert the
non-standard "Return-Receipt-To" header into a DSN request), and
therefore MIMEDefang filters looking for headers such as
"Disposition-Notification-To" or "Disposition-Notification-Options" will
NOT affect DSN.

    Headers such as "Disposition-Notification-To" and
"Disposition-Notification-Options" are associated with the MDN protocol,
and not all MUAs implement it.