When a message gets stuck in your Outbox, the most likely cause is a large attachment.
Choose Send/Receive > Work Offline.
In the NavigationPane, choose 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.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, choose Send/Receive > Work Offline to deselect the button and resume working online.Messages also get stuck in the Outbox when you choose Send but you’re not connected. Choose Send/Receive and look at the Work Offline button. If it’s blue, you’re disconnected. Select it to connect (the button turns white) and choose Send All.