This week, an updated Apple support article offers a reminder on what to do if Mac OS X Mail keeps crashing ("unexpectedly quitting" in Apple-speak). The most likely cause is a "damaged" or otherwise problematic message. In such a case, Mail will initially crash as soon as you select the message and attempt to display it.
Unfortunately, this is only the beginning of your trouble. It gets worse. When you relaunch Mail, it will return to where you left off, again attempting to display the damaged email. Oops. The message will thus cause Mail to crash again, before you get a chance to do anything else. Can you say "endless loop"?
There is a way out of this. The first step is to hold down the Shift key when launching Mail. This forces Mail to open without displaying the crash-inducing message--or any message at all.
Next, drag the horizontal divider in Mail's window so that you can no longer see the area for the display of messages. You can now make a selection in the left-hand column. Only the listing of message titles should display.
Finally, single-click on the message that was active when the crash occurred. Select to delete it. Assuming that the deleted email was the only one causing the crash, your problem should now be over.