How to cure Mail of being stuck on one message?
A corrupted email message can get stuck on your email system, like clogged pipe in your plumbing. In my experience the problem is always one of those joke emails that has been forwarded countless times. 

Web Access To Email

Solution is to use the Web interface to your email provider rather than your email client app. In the web browser, find the offending email and delete. If you really need that message - put it in a folder that doesn’t sync if you can.

Then return to your email client. Your pipes should be clear again. You may need to do a Rebuild command to reconstruct your local cache.