|Disconnect from your internet/network connection, open the Outbox, highlight the stuck email, and move it to the Drafts folder. You can then delete it from there.|
Some things to consider to prevent this in future, check the number of recipients; Outlook doesn't work well with more than 50, and make sure your server timeouts are set to 10 minutes.
"So won’t you give this man his wings
What a shame
To have to beg you to see
We’re not all the same
What a shame" - Shinedown