Cannot send message using server Comcast

I received an email from my alumni that also went to a great many other alumni. Whe I tried to "respond to all," I received a popup that said, "Cannot send message using the server Comcast." and the original email and the popup are frozen on the screens of all my computers. I conntacted Comcast and they do not have an answer. Any ideas?

I can still send and receive emails but the two images are frozen on my computers. I've tried restarting the email and the computer. Nothing yet has worked. Here's what my screen looks like