While addressing other issues I discovered that SpamSieve is NOT quitting when I quit Apple Mail. That is the case even though I just now checked and in SpamSieve’s preferences in the Advanced section, I have the box checked that says “Quit when mail client quits” but clearly it is NOT quitting. I even tried unchecking that box, quitting everything manually, restarting, rechecking that box back to checked, quitting and restarting all again, to test whether it would accept a NEW checking of that preference. But it changed nothing. I know SpamSieve used to quit when Mail quit (but that could have been long ago as I don’t watch that often).
In summary, with both Apple Mail and SpamSieve running, and that above mentioned box clearly checked for “Quit when mail client quits”, if I QUIT Apple Mail, SpamSieve keeps running… Thoughts on why??? FYI, I am running macOS 10.15.2 and SpamSieve version 2.9.38.