Hm.
On your last message, Linc, this is what happens:
Message (which is a reply to a message from me) has my address in the to field, John's address in the cc field, and Mary's address in the from field. When I hit reply all in Outlook, my address becomes the sender, Mary's is the to field and John is in the cc field -- as it should be.
In Mail, what's happening to the same message is Mary gets put in the to field and John's is in the sender, and I'm eliminated altogether!
So, on the server list, I did what you suggested but it has not fixed the addressing issue. It does, however, mean that I'm forced to put in a server, which manually, which at least stops me from accidentally sending stuff from the wrong sender, I think. And I'll have to reinsert the cc stuff. That's an inconvenience that I guess I'll have to live with. I did fill in an Apple feedback form to report this to the developers.
(Now, if I can just get over changing all my other habits... Outlook was misbehaving badly so I switched, but it does have some features I still miss a lot!)