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.
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