[pilif]

On the other hand, by erroneously treating a SHOULD as a MUST, I would say that Google is the one who's not RFC-compliant

[FactolSarin]

Google is rejecting it to ensure incoming messages aren't spam. SHOULD means "you should do this unless you have a really, really good reason not to." Do they have a good reason not to? It doesn't seem so, meaning Viva is in the wrong here.

[davoneus]

No, SHOULD is defined in the RFC, not by colloquial usage. Google is on the wrong, regardless of their "safety" intent.

After all, linguistics is full with examples of words that are spelled the same, but have different meaning in different cultures. I'm glad the RFC spelled it out it for everyone.

[fmbb]

Spam senders don’t have pseudorandom number generators?

[bawolff]

Did i miss the part of the RFC that says google must accept every message? Pretty sure the RFC allows email providers to reject any message they feel like.

[citrin_ru]

RFC cannot force a mail server to accept spam. You may argue that requiring message-id is a bad anti-spam policy but it does reduce amount of spam. In my observations around a half or messages without message-id are spam. I would not use personally this as the only reason to reject a message but I understand why someone may choose to do.

[ragall]

The RFC says a SHOULD is to be treated like a MUST, but well-justified exceptions are allowed.

[alistairSH]

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.

So, it's fairly explicit that the sender should use message-id unless there's a good reason to not do so. The spec is quiet about the recipients behavior (unless there's another spec that calls it out).