[ZWoz]

My take, as a postmaster for hosting company, who don't have any sympathy to gmail (that should be visible from my comments history): Message-ID is absolutely MUST in production e-mails. You can send your test stuff without it, but real messages always have it. Not having Message-ID's causes lot of fun things. All somewhat competent software is capable to add Message-ID's, so lack of it is good indication of poorly made custom (usually spamming) solution.

Rspamd and spamassassin have missing MID check in their default rules, I am sure that most antispam software is same.

[mort96]

Your casual use of the word MUST is not the same as a standard document's use of the word MUST. Your real world experience is entirely irrelevant to the conversation about what the standard requires.

[stefan_]

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. One lesson surely most everyone learns in CS is that unique identifiers are maybe unique to the system generating them, but to rely on foreign generated IDs being unique globally is a terrible idea that will break within the minute.

So at that point the ID has no value to me except being obliged to carry it around with the message, so maybe the originating system can at some point make sense of it. But then there is obviously no reason to ever reject mail without it, it's an ID valid for the sender and the sender didn't care to include one, great, we save on storage.

[jasode]

>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

Your 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...

[ZWoz]

MID-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.