Do you have windows xp, vista, or 7?
So you've tried to delete it in Outbox, but it did not work.
In Outlook, please click on File menu on the top, then click on Work Offline (or click on the Work Offline icon if available), then see if you can delete the stuck message in Outbox.
, then you can uncheck Work Offline
If that does not work, close Outlook, then click on Start and search for the phrase:store:mapi outboxand press Enter, then look for the stuck message and delete it.
(Actually, the Work Offline option is under the Send/Receive option in Outlook)
If that still does not work:
Please scroll up/down this web page to see all my suggestions. Please let me know if there is still any issue. Thank you.