• 1. Close your Outlook / Mail program
  • 2. Open your Outlook / Mail program back up
  • 3. Click on Outbox right away before the program tries to send the message
  • 4. Click on the message and delete it immediately

Send mail stuck in your Outbox, or delete it. The usual cause for stuck mail is a large attachment. 

For video instructions, you can check out the step-by-step video here.

Fix messages that are stuck in the Outbox

  • 1 - Click SEND/RECEIVE > Work Offline.          

  • 2 - In the navigation pane, click Outbox.             

  • 3 - 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).

  • 4 - 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.            
  • 5 - After Outlook closes, start it again and repeat steps 2-3.             

  • 6 - After you remove the attachment, click SEND/RECEIVE > Work Offline to deselect the button and resume working online. Messages also get stuck in the Outbox when you click Send, but you are not connected. Click SEND/RECEIVE and look at the Work Offline button. If it is blue, you are disconnected. Click it to connect (the button turns white) and click Send All.

Source: MS Office Support