MacOS X Mail says it is locked

If MacOS X Mail (Mail.app) should crash or your machine loses network connectivity, Mail might say it cannot open your mail as it is locked.

To fix this, you can (after making sure Mail.app is closed) delete the two lock files from your home folder that control the locking.

Open a Finder window (click the blue 'face' icon in the Dock or double-click your hard drive icon)
Go to your home folder (has the same name as your username - looks like a house)
Double-click the folder Library
Double-click the folder Mail
Drag these files to the trash: Envelope Index.lock, Envelope Index-journal
Start Mail