Had a major problem with mail.app this morning.
One of the emails in my inbox was an invitation to some social-networking-site, in html. And as soon as mail.app tried to display it, the app would crash. This was the currently selected message, so I never even got a chance to “fix” it before it would crash. Solution?

Hold down the shift-key when launching mail.app to prevent it from highlighting any mailboxes. Of course, as soon as I selected the inbox, it would crash again. The solution was to also collapse the preview-pane before selecting the inbox, and then selecting (and deleting) the corrupted message.

Note this down for future use:) Seems to me it’s a quite common bug with html messages, judging by various Mac forums.