I’ve seen this issue a lot with my clients and, after having done a bit of research, this link shows you just what can be done to get these stuck messages sent or deleted.
Also, if you are sending a message with an attachment that is 20MB or greater…this, too, can get stuck in the outbox. Sometimes, they keep resending dozens of times and STILL show in the outbox. If this happens to you, this can be due to the “email scanner” within your antivirus suite OR you may have an add-in within your version of Outlook that needs to be disabled (ie. Virus Scanner Integration add-in).
If this does not work OR if you do not have an add-in within your Outlook pertaining to email scanning…then the link above may be your best answer. I found that, in Outlook, choosing to work “Offline” and then going in to select and delete the message that is stuck in the outbox works best.
If you have any questions, stop on by my contacts page and send them my way.
Happy Computing!! 🙂