The answer is here, but I wanted to post with something that would catch Google searches, since none of mine today worked until I was trying to delete Mail.app all-together to re-install it (which you can’t really do).

But, just run:

killall Mail &> /dev/null; mv ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist ~/Desktop && open -a Mail

If it works, delete the com.apple.mail.plist file on your Desktop!

…it worked on my machine 😉

I did have to re-set all my Mail.app settings again, but worth it!