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.
If you have a large message stuck in the Outbox and receive an error message when you try to open or delete it, you need to set Outlook to work offline. 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. Use Shift+Delete to delete it permanently, especially if you have an IMAP account as it will create an error trying to sync to the server.
If you want to keep the message but remove the attachment, open the message and remove the attachment then Send.
Work Offline is on the Send & Receive ribbon. The screenshot shows the default setting of On (as in Working Online). When Outlook is Offline, the button is highlighted and Working Offline is in the status bar on the lower right.
After the stuck message is deleted (or the attachment removed), set Outlook back online.
If your version of Outlook doesn't include a Work Offline command, or you are having problems getting Outlook into offline mode, 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. You can close and reopen Outlook after setting it to work offline, if you are in a hurry.