Apple’s Mail Locked Up?

Happened to me this morning. The symptoms are: launch mail, click anywhere in the windows and watch the spinning pinwheel mock you constantly… spinning and mocking you. If Mail on your Mac hangs or stops responding then here’s a 1 minute fix that may or may not work for you. Backup and delete (just drag them out of their locations to another folder or your desktop) the following:
~/Library/Mail/Envelope Index

(The tilde “~” is shorthand for your home directory or /Users/YourUserName)