Using New Outlook on Windows for professional communication and productivity
This worked for me,
Click on the Send receive at the very top bar
Click on Work offline
Got to draft and delete it or click on the garbage can
Click on Send receive
Click on Work offline. This will set it to Work online again.
Hope this helps you!
I don't think this suggestion is particularly helpful, I've looked at Outlook (the forced rollout to replace Mail) and there isn't a Send/Receive at the very top bar. Which version of Outlook does this relate too?