gmail is stuck on "cannot send message using the server ".

gmail is stuck on "cannot send message using the server ". I can't delete the stuck messages.

It is being sent to multiple recipients so I think that is the problem - too many addresses. I can't even get the screen to scroll down to the bottom of the error message to see if there's a Cancel or Delete button at the bottom. How do I clear these out of the system? I tried force quit, update, power off and on…