There is a "simpler" way. The fix:
- Disable your network interface(s) (Windows+W, type "view network connections" and press Enter, right-click and Disable your network connection)
- Open the Mail app and go to the Outbox
- There is now a delete icon in the top right for each stuck mail item !
- Don't forget to enable your connection(s) again
Also another option is:
- Go to your desktop
- Right-click on your wireless in the System Tray
- Go to Open Network and Sharing Center
- Go to View Network Connections (on the left)
- Disable from there and then follow step 2 above