All of us have experienced a moment when we've pressed "Send" only to realize that it was going to the wrong individual -- or that we've hit "Reply All" to about 80 people. But if you keep a cool head (and some fast fingers) there is a way to stop the message before it heads out.
The Microsoft Mac Support page has a nifty article that essentially guides you through canceling the process. Briefly: (Command + 7 to get the Progress window, then click the gray X next to the progress bar ), then switching Outlook into "Work Offline" mode (click the Outlook menu and select Work Offline) so you can hunt down and modify/kill the message before it departs. The Outbok that dispeared a few updates ago appears in offline mode at the bottom of the navigation pane.
It's easier on Windows: File->Work Offline, then go to the Outbox and delete/modify the message before switching back to normal mode.
More details are on the Microsoft Mac Help site: