Mail App Frozen Due to no preferred outgoing server

My mail app has been unusable for the last several months with the message that hundreds of messages (mostly calendar notifications that are YEARS old) could not be sent because the account does not have a preferred outgoing mail server. I tried deleting all of the accounts, and reinstalling them...that didn't work either. There are so many messages piled up on top of each other that i cannot see them all, much less resolve them. Is there an easier "batch method" to clear them?