> Google might be too strict in its filtering

they are, but not in this case

Message-Id being basically required for _automated_ mails (very similar mail send to a lot of people) requiring Message-Id is a de-facto industry standard. Sure some providers don't care and some might make it just more likely that your mail ends up in spam. But this could have happened with pretty much any mail provider widely used by companies.

much more funny (/s) is if you start out in a startup and still use the default template for password reset/on-baording links of a widely used system (e.g. Keycloak) and it turns out multiple larger but "cheap" phishing campaigns did the same and now MS/Google and other suspect you are running a phishing operation

or when you use a local data center and can't send mails to MS/Outlook anymore because it turns out someone did some legal questionable things on them and MS wanted the personal information to be handed over _without court order or any ongoing legal proceeding against this people_ and they didn't hand it over (partially because they legally aren't allowed to...) and MS decided to retaliate by permanently blacklist the IPv4 range of that data center(s) which just happen to locally compete with Azure while self hosted mails competes with Outlooks...