Reset Mail App Preferences


I've managed to confuse the mail application by first setting up an IMAP account and then deleting it, then setting up a POP3 account. Although I can receive messages OK from the POP3 account, the app fails to send messages because it is somehow still looking for the deleted IMAP account. Is there a text preference file I can either edit or delete to "reset" the email app?
Thanks for any help you can provide!