alt Hacker NewsThe official definition of SHOULD per RFC2119:
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.
Replies
You assume that internet standards are prescriptivist; that the document describes how it is to be implemented. In practice it's often descriptivist, with the standards documents playing catch-up with how things are actually going in practice.
Anyway, in general you can expect that doing unusual but technically valid things with email headers will very often get your messages rejected or filtered as spam.
For producers, ignoring a SHOULD is riskier because it shifts the burden to every consumer.
For consumers, ignoring a SHOULD mostly affects their own robustness.
But here Google seems to understand it as a MUST... maybe the scale of spam is enough to justify it. Users are stuck between two parties that expect the other to behave.
Google interpreted it that way because it drives more people to use gmail.
You can argue that you not obligated to use message-id but if you don't use it you should blame only yourself that your messages are not accepted. In requiring message-id I would side with google (though in general I think they anti-spam is too aggressive and lacks ways to report false positives). Full RFC compliance (as in not only MUST but also SHOULD unless you have a very good reason) is the easiest part of making sure your emails will be delivered.