Hi Eric in Oakland !
Re: "Messages from a Previous Sender, already in Contacts, still go to Junk
Both Previous Senders and Contacts are both set to be exempt from Junk filtering in Mail settings,
but messages from one particular sender who meets both these criteria are still repeatedly sent to Junk".
Wondering if the messages are deemed "junk" by your eMail Provider, before they reach the "Mail" app.
You may want to log-in to your eMail provider's website online to check the settings that are in place there.
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Else:
You may want to send feedback to Apple. They will likely not reply, yet your feedback will reach those who work behind-the-scenes, and can make changes.
Web-form for feedback:
Feedback - Mail - Apple
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Re: (And why in the world is there no topic category for Apple Mail?!)
Seems we would need iOS Mail, Mac Mail, iPadOS Mail, WatchOS Mail ... to prevent confusion?
There is a Mail User Guide for Mac available: Mail User Guide for Mac - Apple Support
and
Mail Support Website : (info. / support / contact)
Mail - Official Apple Support
All the best :-)