Who exactly generates new Message-ID per each transmission???
There are legitimate mailing list / bulk mail applications in use by
Fortune 500 companies that only keep a message template on disk. It's
the only way to build an application capable of running on a single
machine and sending personalized versions of a single E-mail to tens of
millions of people. All the "variable" information, like the recipient's
name, E-mail address, date, and the Message-ID field, are added on the
fly while streaming the message to the SMTP receiver. If the message
gets a 4xx on the first try, the MTA logs the attempt but throws away
everything else; so indeed the Message-ID changes on each attempt.
No, I don't remember which applications did this, just that I saw it a
lot at Habeas. And yes, it does play merry hell with duplicate detection
(and on message tracking -- the Message-ID field is nearly useless).