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Peter, you might look for a lack of successful DKIM signing on messages delivered to Fastmail but not making it to Gmail. Google might block messages which SPF hardfaiil if they have no other reputation features.
If a Google rejects them at the SMTP acceptance stage, I would assume that if SRS was used that Fastmail would in form the sender (not the FM account). Do you know if any sender receives a message disposition notice from Google?
If you can come up with some relatable test that fails, Fastmail could examine their logs to see how the Google receiving server is responding.
Unfortunately, I can find no currently maintained list showing SPF and DKIM policies for various email services.
Bill
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