Chosen Solution
Mail will not open or shut down. Tried shutting computer down and it says mail is canceling shutdown.
Have you tried force quit the mail app? What happens when you try to open the mail app? The icon just jumps like its opening but nothing comes up? To force quit, when you click on the mail app icon you will see your finder bar changes from finder to mail, click the apple icon and find force quit in the menu, select the mail app and tell it to force quit. I think the key combination to force quit is Command+Option+Esc
