Mail has gone spastic

I just upgraded from Mavericks to Sierra. Everything was working fine in Mavericks and most things are working ok in Sierra, but the very first time I tried to reply to a message in Mail it went crazy. Whenever Mail is active the message I tried to send is jumping on the screen alternating with the Mail window a couple times a second. I can do nothing with it except quit Mail.

I hope someone can help!

I am working on a 15" mid 2012 MacBook Pro.