We've all done it once or twice -- we 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 already being transmitted.
Use Desktop Search
If you use Windows 7, you can use the Start menu's Search field to find and delete messages stuck in the Outbox. You need to close Outlook to delete the message.
In preliminary tests, it appears that this method might delete messages from a large pst file that errors when you try to delete the items from within Outlook.
To use this method, close Outlook then type outbox or store:mapi outbox in the Start search field. Wait for search to finish then find the message and delete. Messages deleted using this method bypass the Deleted Items folder and are permanently deleted.
Note: the search method does not work on Windows 8/10.
You can delete the stuck message if you set Outlook offline using the File, Offline menu. Wait about five minutes or more for the mail spooler process 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.
In Outlook 2010, Work Offline is on the Send & Receive ribbon. The screenshot shows the default setting of On. When Outlook is Offline, the button is highlighted in orange.
If your version of Outlook doesn't include a Work Offline command, disconnect from the Internet or disable the network card in the Control Panel, Network. Wait a few minutes, longer if the stuck message is really large, then delete it.
This is one time when patience is a definite virtue - make sure you give Outlook plenty of time to stop processing the message before trying to delete it. A very large message may require longer than 5 minutes to stop processing.