Just wanted to throw in my experience - Mail suddenly started crashing constantly, I couldn't keep it open for more than a few seconds. Rebooting didn't help, home & power button restart didn't help, airplane mode was fine. I sync my mail with Thunderbird on my laptop as well, so I took a bunch of messages I'd received since the last time Mail seemed to be working, and moved them to a local offline folder - that did the trick. A few of them were bounced messages that some server had fed back at me - I suspect one of them had a wonky header and Mail was just freaking out at it.
Not sure what you'd do if you don't have another computer to sync to, though... I guess find another computer, go into Webmail, and clear out the suspects.