macOS Mail won't clear out the outgoing SMTP server list
Thank you to Andreas at Apple support for this excellent answer:

counter-intuitively: the &quot;Automatically manage settings&quot; button for the OUTGOING is (mysteriously) TIED TO THE SAME BUTTON for the INCOMING SETTINGS.

If you are having trouble unchecking the &quot;Automatically manage settings&quot; on the OUTGOING section, you must first uncheck the same setting on the INCOMING section.

All the SMTP servers should have unique settings. If you try to save another outgoing SMTP server with the same domain, it will detect it and complain

If you try to use (-) to delete a SMTP server off the list, be sure it has NO ATTACHED mailboxes to it. Quit &amp; restart Mail to see the item removed, and recheck to confirm it is no longer there. (Sometimes it comes back, even if you follow these steps. However, I was able to remove all of the ones I wanted removed by repeating the steps. Once it is truly removed it appears not to come back anymore.)