Nothing new here, but I seem to need this often enough to post it here also. When someone tries to send a large file in Outlook, it has a tendency to get stuck in the Outbox. Do the following to fix it within minutes:

First, try setting Outlook offline using the File, Offline menu. Wait about a minute or so or close Outlook and reopen.

If you are unable to delete the message while in offline mode or cannot go into offline mode, you’ll need to change your default delivery location.

The following steps are for Outlook 2002/2003, the steps for older versions are the same, although the menus are different.

1. Add a new PST using the File, New, Outlook Data File menu.
2. Open the Email Accounts dialog on the Tools menu.
3. Select view or Change existing email accounts and click Next.
4. Select the new personal folders file from the Deliver new email to the following location list.
5. Click Finish and restart Outlook.
6. Show the folder list, using Ctrl+6 if necessary, and find the old Outbox. Delete the message.
7. Repeat steps 2, 3, 4, and 5 to restore the original pst as the default delivery location.
8. Show the folder list and move any new messages from the new pst to the original pst.
9. Right click on the new folder’s name and Choose Close to remove it from your profile.

Ripped off from Outlook-Tips.net