I have come across this issue for years in Outlook 2000, 2003, 2007, 2010 when an email can not be sent no matter if you try to hit send receive, reboot, etc. its not too big and email address are ok yet it is stuck in the outbox.
Turn off/unplug your network connection and once Outlook is offline (sometimes needs to close outlook and or reboot the PC) then go to your Outlook outbox and delete the item. Connect to your network again. Go to your deleted folder and open up the messages you deleted and resend it.