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!