alt Hacker News> 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 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).
Replies
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.
> 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.
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.
> 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.