There are times when an email will not send and gets “stuck” in the outbox using Outlook. Usually this is due to either a corrupt email or an attachment that is too large to send by email. In either case, the solution is the same.
You want to change Outlook to work offline. Click on File and then select Work Offline. Close Outlook altogether. Get to the Task Manager and make sure Outlook is not running as a Process. If it is, then End Process on it. To get the Task Mgr. hold down Ctrl + Alt + Del keys altogether and once. Click to start the Task Mgr. Find Outlook running under the Processes tab, highlight it and click End Process at the bottom of the window.
Now open Outlook again. Go to the outbox and either click and drag the email to another folder, (Saved Mail for example) or right click it and select Delete. Do not try to send it as the option to Work Offline is active.
Once moved or deleted, then click File and uncheck Work Offline. Close Outlook, End Process in the Task Mgr. again and then restart Outlook. The email should be out of the outbox and the Send/Receive feature should work as normal.