When a message gets stuck in your Outbox, the most likely cause is a large attachment.
Click Send/Receive > Work Offline.
In the Navigation Pane, click Outbox.
From here, you can:
Delete the message. Just select it and press Delete.
Drag the message to your drafts folder, double-click to open the message, delete the attachment (click it and press Delete).
If an error tells you Outlook is trying to transmit the message, close Outlook. It may take a few moments to exit.
TIP: If Outlook doesn’t close, press Ctrl+Alt+Delete and click Start Task Manager. In Task Manager, click the Processes tab, scroll down to outlook.exe, and click End Process.
After Outlook closes, start it again and repeat steps 2-3.
After you remove the attachment, click Send/Receive > Work Offline to deselect the button and resume working online.
TIP: Messages also get stuck in the Outbox when you click Send but you’re not connected. Click Send/Receive and look at the Work Offline button. If it’s blue, you’re disconnected. Click it to connect (the button turns white) and click Send All.