Just a thought. Try dragging the Mail .plist on your desktop while mail isnt open, and once you have open Mail again and try again.
You might have a corrupt Preference file, and you will either locate it username/Library/Containers/com.apple.mail/Data/Library/Preferences
or
/username/Library/Preferences/ but im more inclined to check the first Path. If you drag it to the Desktop and it works for you, trash it, but if not you can put it back from where it came from.
Also try to Verify Permissions via Disk Utility and the Repair.
Open DU sélect your HD in the left column, select First Aid tab and Verify, and then do the same to Repair. Log out then back in and see if that makes any difference.
Cheers