Answer
This question was answered on December 12, 2005.
Much of the information contained herein may have changed since posting.
An error dialog stating the MAPI sppoler has begun sending the message ususally means that it was too large for your mail server.
Try setting Outlook offline using the File, Offline menu. Wait about a minute and close Outlook, then reopen.
If you are unable to delete this message in offline mode, or cannot go into this mode, you will need to change your default delivery location.
Here are steps for Outllok 2002/2003. These steps can also be used on older versions such as 2000. The menus may be different though.
1. (i/sutck 4jpg) 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. (i/stuck 6jpg) 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 sthe folder list and move any new messages from the new pst to the original pst.
9. (i/stuck 8jpg) Right click on the new folder's name and Choose Close to remove it from your profle.