> If it's something else, I'm sorry I just don't understand.

I'm trying to explain to you that you're speaking very authoritatively about how things were done 30 years ago but things have changed since then, Gmail won't even send a message non-delivery notification to non-DKIM hosts.

Same thing with the g-msgid I'm telling you about – google documented it explicitly as a unique identifier, see here for example: https://developers.google.com/workspace/gmail/imap/imap-exte...

So yeah, things changed.