When you'reĀ online and email works normally, you won't see the Outbox. The Outbox appears at the top of the sidebar only when messages are waiting to be sent.
Tip
If you don't see the sidebar, click View > Sidebar.
If email continues to sit in your Outbox, you might be offline. To check, select the Tools tab, and move the slider to Online.
Note
The Online/Offline toggle described in this article is not available in New Outlook for Mac.
See Also
Cannot send my email message