A few times I have had an outgoing message stuck in my outbox that tries to send everytime I launch the Mail.app. It would sit there and try and try and not let me do anything, so all I could do was force quit the program. I finally figured out how to work around this.
I navigated to ~/Library -> Mail -> Mailboxes and control clicked on the Outbox.mbox file and selected "Show Package Contents." I then moved the file named 'mbox' out of that window and to my desktop (or trash if I did not need it). Then I could launch Mail and actually make a change to prefs or whatever I needed to do.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030115073827767