We're not receiving emails from a specific address, not in regular mail, not as junk, etc. Sender says mail has been delivered successfully, but I don't receive anything from this specific sender.
Is it possible your mail service provider is somehow filtering out this sender or the sender's domain as a spammer/abuser?
(Erroneously, of course!)
I don't think so, since i do receive these emails in my gmail account or corporate account. It seems @mac.com filters it out...
Maybe this Apple support article will help:
Mail – Troubleshoot Send and Receive
This link has many more troubleshooting articles:
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