Cannot Delete Unsent Message

I tried to send a message in Windows Mail with too large a file attachment. The message cannot be sent because the attachment is too large. Every time I close Windows Mail I get the message: "You have unsent messages in your outbox. Do you want to send them now?" If I answer "Yes," I get the same error message. The message is no longer in my Outbox so I can't just delete it. I get the same error whenever I close Windows Mail. I have rebooted, but that has not helped. Any advice will be greatly appreciated.