When attempting to send a message which is too large for your mail server, it may become stuck in the outbox, unable to delete it, because Outlook says it has already started to send the message.
If this happened to you, there are two things to try...
Try setting Outlook in offline mode:
If this solution didn't work for you, then the below one might just do the trick; it's a bit more complex, though.
Another solution is to change the default delivery location. This way, Outlook will stop sending messages from the current Outbox, thus enabling you to delete the oversize message.
The following steps are the same for Outlook XP and 2003, although the menus may be a little different: