> isn't a valid excuse to reject a client either.

Yes it absolutely is: https://www.rfc-editor.org/rfc/rfc2119 is quite clear.

    3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
       may exist valid reasons in particular circumstances to ignore a
       particular item, but the full implications must be understood and
       carefully weighed before choosing a different course.

If the client SHOULD do something and doesn't, and your server does not know why, you SHOULD disconnect and move on.

If the server has considered fully the implications of not having a Message-ID header, then it MAY continue processing.

In general, you will find most of the Internet specifications are labelled MUST if they are required for the protocol's own state-processing (i.e. as documented), while specifications are labelled SHOULD if they are required for application state-processing in some circumstances (i.e. other users of the protocol).

▲ Dylan16807 an hour ago | parent | next [-]

> If the client SHOULD do something and doesn't, and your server does not know why, you SHOULD disconnect and move on.

That is not a rule.

In this situation the server can reject any message if it wants to, and not doing a SHOULD tests the server's patience, but it's still ultimately in the "server wanted to" category, not the "RFC was violated" category.

	▲	geocar 13 minutes ago \| parent [-]
		You are confused. The RFC is a request for comments. The specific one in question is about Internet Mail. If server implementers want their mail to be delivered these are things they SHOULD do. That's it. It isn't something you can give to your lawyer, and nobody cares about your opinion about what you think "should" means you can make someone else do. This is how it is.

▲ buran77 39 minutes ago | parent | prev | next [-]

> If the server has considered fully the implications

The server "considers" nothing. The considerations are for the human implementers to make when building their software. And they can never presume to know why the software on the other side is working a certain way. Only that the RFC didn't make something mandatory.

The rejection isn't to be compliant with the RFC, it's a choice made by the server implementers.

▲ drecked an hour ago | parent | prev | next [-]

That clearly means it’s not required.

How does Google know whether or not the sender has a valid reason? They cannot know that so for them to reject an email for it means they would reject emails that have valid reasons as well.

	▲	geocar 16 minutes ago \| parent [-]
		> That clearly means it’s not required. You and I have different definitions of "clearly" It is not required for the protocol of one SMTP client sending one message to one SMTP server, but it is required for many Internet Mail applications to function properly. This one for example, is where if you want to send an email to some sites, you are going to need a Message-ID, so you SHOULD add one if you're the originating mail site. > How does Google know whether or not the sender has a valid reason? If the Sender has a valid reason, they would have responded to the RFC (Request For Comments) telling implementers what they SHOULD do, rather than do their own thing and hope for the best! Google knows the meaning of the word SHOULD. > it means they would reject emails that have valid reasons as well. No shit! They reject spam for example. And there's more than a few RFC's about that. Here's one about spam that specifically talks about using Message-ID: https://datatracker.ietf.org/doc/html/rfc2635

▲ hsbauauvhabzb 3 hours ago | parent | prev [-]

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 35 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 2 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.