It’s not unusual for messages to get stuck in the Outbox. Most of the time they are too large so the SMTP server won’t accept them but Outlook keeps trying to send them. You can’t delete, open, or move them because Outlook is in the process of sending them: “Can’t open this item. Outlook has already begun transmitting this message”

To delete these messages, first try setting Outlook offline using the File, Offline menu. Wait a few minutes (up to 5) for Outlook to release the message before trying to delete it. If you still get the error, you’ll need to change the default delivery location, delete the message, then set the default delivery location back to the original location.

See http://www.outlook-tips.net/howto/stuck_message.htm for the step-by-step to change the delivery location.