Either the server must explicitly confirm to servers or the clients must accept everything. Otherwise message delivery is not guaranteed. In the context of an email protocol, this often is a silent failure which causes real-world problems.

I don’t care what the protocol rfc says, the client arbitrarily rejecting an email from the server for some missing unimportant header (for deduction detection?) is silly.

▲ behringer 2 hours ago | parent [-]

If it was unimportant it would be MAY.

▲ hsbauauvhabzb 37 minutes ago | parent [-]

Is the server somehow unable to inject an ID if the sender did not send one? Stop hiding behind policy and think for yourself.

	▲	geocar 4 minutes ago \| parent [-]
		> Is the server somehow unable to inject an ID if the sender did not send one? Yes. https://www.rfc-editor.org/rfc/rfc2821#section-6.3 refers to servers that do this and says very clearly: `These changes MUST NOT be applied by an SMTP server that provides an intermediate relay function.` That's Google in this situation. > Stop hiding behind policy and think for yourself. Sometimes you should think for yourself, but sometimes, and friend let me tell you this is one of those times, you should take some time to read all of the things that other people have thought about a subject, especially when that subject is as big and old as email. There is no good reason viva couldn't make a Message-ID, but there's a good reason to believe they can't handle delivery status notifications, and if they can't do that, they are causing bigger problems than just this.