So add a message id at the first stop, or hard ban the sender server version until they confirm. A midway point that involves a doom switch is not a good option.
reply> So add a message id at the first stop
replyThat should have already happened. Google is not the "first stop".
> hard ban the sender server version until they confirm
SMTP clients do not announce their version.
Also I don't work for you, stop telling me what to do.
> A midway point that involves a doom switch is not a good option.
No shit. That's almost certainly a big part of why Google blocks messages from being transited without a Message-ID.