i have a mail problem similar to yours in a different thread here. let me tell you some things that might help you.
first, set up a new test user on your system and set up mail for him. if it works, we know the problem is in the mail plist in your users library and you can delete it, set yourself up again and all will be fine.
second, there is the possibility that your ical.app todos are corrupted and this can cause mail to fail to execute. you might want to delete all ical todos and then run mail.app
third, there are better minds than mine here. to help you such people will need to see the crash report from mail in your user library. you can post it here.
best regards,
marc