Step1: Click Send/Receive > Work Offline.
Step2: In the Navigation Pane, click Outbox.
Step3: From here, you can:
Step4: Delete the message. Just select it and press Delete.
Step5: Drag the message to your drafts folder, double-click to open the message, delete the attachment (click it and press Delete).
Step6: 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.
Step7: After Outlook closes, start it again and repeat steps 2-3.
Step8: After you remove the attachment, click Send/Receive > Work Offline to deselect the button and resume working online.