Today our system administrators updated the Let's Encrypt SSL certificates on servers in the US datacenter. As a result, you may receive SSL-related warnings in your mail programs. The solution to remove these warnings is to update the mail server in your mail program/app. Please change the incoming/outgoing mail server from:
mail.yourdomain.com (or similar)
You should be able to find the "servername.com" part in the warning itself, as "ssl.servername.com".