Am 25.06.2013 15:28, schrieb Timo Sirainen:
Also there are several potential problems.. Like if there are duplicate Message-ID: headers, but the body is different, should that be a duplicate?
the answer is simply *yes* because there must not be the same Message-ID's for different messages because the words "single unique message identifier" are pretty clear
RFC2822
Though optional, every message SHOULD have a "Message-ID:" field. Furthermore, reply messages SHOULD have "In-Reply-To:" and "References:" fields as appropriate, as described below.
The "Message-ID:" field contains a single unique message identifier. The "References:" and "In-Reply-To:" field each contain one or more unique message identifiers, optionally separated by CFWS.
these days "every message SHOULD have a Message-ID:" is outdated
we started many years ago to block *any* message missing the header because every sane SMTP implementation adds it if it was missing from the client and so only broken implementations which are mostly spammers would be affected