>Why? If I'm writing a mail receiver, and I'm told there is some unique ID generated by the sender in a loosely specified way, the first thing I'm doing is ignoring that value forever. [...] So at that point the ID has no value to me
replyYour framework of analysis is based on someone else's database key ids being irrelevant to you. That's true.
But another framework of analysis is tracking statistical correlations of what spam looks like. Lots of spam often don't have message ids. Therefore it's used as a heuristic in scoring it as potential spam. That's why other postmasters even without SpamAssassin independently arrive at the same answer of trying to block messages without a message id. Example: https://serverfault.com/questions/629923/blocking-messages-w...
Ah, so it's just the evil bit, or lack of.
replyMID-s are used by MUA-s for referring earlier messages, tracking answers and so on. So any software expecting dialog (messages coming back) needs to deal with MID-s correctly. Missing MID-s show that said communication is one direction, because broken dialog has not been problem.
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