Next thing it to rebuild the preference file. I'm not a Mac user any longer, so I had to search for the instructions which I found here 11 easy ways to fix Apple Mail not working
In Finder, click Go > Go to Folder
Enter the following file path and press the Return key to go there: ~/Library/Containers/com.apple.mail/Data/Library/Preferences
After that, drag the com.apple.mail.plist file to your Desktop to reset the preferences
Now reboot your machine and reopen the Mail app
If Mail still doesn’t work properly, move the plist file from your Desktop back into the Preferences folder to restore your preferences.