Message stuck in Outbox
It doesn’t happen too often but when a message is stuck in the Outbox it can be a real pain.
Open the message from the Outbox and close it again. Now select the message and press DELETE.
Put Outlook in offline mode. To do this go to File-> Work Offline. Now restart Outlook and see if you can delete it (you can try this in combination with method 1). Don’t forget to put Outlook back On-line again.