certificate of email server is invalid

I have a Verify Certificate pop up on my iMac computer. I cannot close this pop up window. I am unable to quit email. I clicked on an email link yesterday and now I can't close the email form that is trying to connect to my email server. My desktop is becoming cluttered with windows I can't close and email I can't quit. What do I do. This became an issue after a recent update. Suggestions?