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.
I just took a good look and ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist is most definitely what used to be ~/Library/preferences/com.apple.mail.plist. When you look at it in Xcode it isn't that fearsome, it is a typical property list file but there are a lot of "Dictionaries" and some of those "Dictionaries" have up to 30 items, and I would have sworn there was no way it would have ~5500 lines, but when I opened it in BBEdit the way it was parsed there are 5,847 lines as in many cases each word is a separate line and there are a bunch of formatting tags. But it is definitely the same file that used to be in ~/Library/preferences/com.apple.mail.plist with a few additions for new Mail features.
I don't recall if plantsower uses Time Machine or not, but if she does a possible solution would be to restore ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist from a point in time prior to the start of her problem. If that did not correct the problem or other problems arise, then it would be a simple matter to restore the most recent version of the file from Time Machine.
ASIDE: 🔆 I don't know why I hadn't though of using Time Machine in similar situations before.💡 Old habits I guess. 👴ðŸ»
(A major part of the problem with troubleshooting vintage OSs is that we can't access them.)
AMEN!
