Artie, this may be a red herring as I am using MacOS as you are, but I did a search for com.apple.mail.plist using FindAnyFile and to my surprise found several on my boot drive. Then I started opening them in Xcode and found the contents were very different and the one in my user Library was almost certainly NOT the property list plantsower is looking for. I found the one I believe you all are looking for is:
~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist
When and why Apple has moved it and why they have chosen to use such a baroque file path is well beyond my ken, but there it is. You have done a great job guiding plantsower so far and I am backing out and leaving it up to you to determine if that was the case in Sierra or a more recent change.
PS: That isn't the only thing that has migrated to a new home in the ~/Library/Containers folder.
I think this is reverse deja-vu, because if I remember correctly, I called your attention to the container arrangement in another thread a while back, and like you, I haven't got a clue why Apple did what they did. It's buried in obscurity.
I've run into a wall in that the oldest file system I'm able to examine is High Sierra, and Apple had already instituted containers by then. (I tried to examine Sierra in Pacifist, but it was a useless exercise.)
And finally, ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist has got
~5,500 lines of text in both High Sierra and Catalina, so I wouldn't mess with it under any circumstances even if it's the same plist that Rita's got.
