try moving the Mail plist from your user account's Preferences folder and quickly setting up your email again to see if it crashes.
if it still crashes quit Mail and place the plist back to be back where you where before.
Seems to do the job, but I haven't set up all my accounts yet. It's just strange that it started to crash all of a sudden, no changes at all.
Had the same problem unfortunately. I deleted EVERYTHING from the mail folder in the library and let it rebuild, seemed to work.