Sometimes a message can get 'Stuck' in your Outlook Outbox. Most frequently it happens when you try to send a very large message.

Occasionally Outlook will not allow you to delete such a message. In this case, one of the following methods may work:

1. Open the message from the Outbox and close it again. Now select the message in the Outbox and press DELETE.
2. Put Outlook in offline mode. To do this go to File-> Work Offline. Restart Outlook and see if your message will delete now. (You can try the first way again). Remember to switch back to Online mode again.
3. Create a new pst-file: File-> New-> Outlook Data File… Set this pst-file as your default delivery location in your Account settings; Tools-> E-mail Accounts-> Button Next. Restart Outlook and your previous pst-file should show up as the secondary set of folders (if not, open it) and you should easily be able to remove the file from the old Outbox folder (or you can delete the entire folder). Set the original pst-file back as the default delivery location again and restart Outlook; the Outbox folder will be recreated and will be empty.
4. Use the Microsoft MDBVU32.EXE tool to forcefully clear the Outbox (Experts Only).

If using methods 3. or 4. then you should back up your old PST file first (don't say we didn't warn you).

The following articles may also help:

KB291613: Sent Messages Remain in Your Outbox
KB293031: Message Copy Remains in the Outbox When You Send Through a Hotmail or IMAP Account While Offline
KB311001: You Cannot Send Messages if There Are More Than One Thousand Messages in Your OST Outbox
KB293038: A Copy of a Forwarded Message Stays in the Outbox
KB195922: OL2000: How to Troubleshoot Mail Stuck in the Outbox