Google's Postmaster Tools site has a "Report deliverabilty issue" link at the bottom left navigation column.

https://postmaster.google.com/v2/sender_compliance

it isn't a google bug

Message-Id being required for automated mails is a de-facto industry standard

while the consequences differ between mail provider, it missing will also make it much more likely for mail to be reject or put into the spam folder

It's also well known. Pretty much viva engineers fucked up doing proper research.

Now to be fair:

- it sucks that you can't just implement the RFC(s)

- the standards suck, docent of different RFCs overlapping and replacing each other and referencing often older versions of other RFCs, with docents of ways to do the same things of which only some can be used reliable in practice and a common gaps in the standards about edge cases or about the "higher level semantics" of constructs.

- so overall mail seems very simple at first but if you want to automated send mails reliable internationally it's a total pain and Message-Id is just the head of the iceberg.

Which is, of course, hidden behind a login wall