Okay, I see what you mean. That information is stored in Mail's preferences. You can trash that file, but you'll lose all your preferences and will have to start over. However, you will not lose your stored mail or mailboxes as you'll be able to import them later.
If you still want to proceed, the file is here:
/Users/your_user_name/Library/Preferences/com.apple.mail.plist
Trash this file, then launch Mail.app and set it up.