Delete large messages stuck in your Outbox

We've all done it once or twice -- we'll have a message stuck in the Outbox because it's too large for Outlook to send. Deleting it doesn't work - Outlook returns an error because the message is being processed by the MAPI spooler

In most cases you can delete the stuck message if you set Outlook offline using the File, Offline menu. Wait about a minute or so for the spooler to shut down or close Outlook and reopen. Go to the Outbox and delete the message then put Outlook back online using the File, Offline menu.

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.

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.

Browse to the old message store, delete the message from the Outbox and repeat steps 2 - 5 to restore the original PST as the default delivery location and close the new PST.

More Information

Delete a Message Stuck in the Outbox Includes screenshots.  (Outlook-Tips.net)