Are you able to retrieve Mail? Do you get a dialog asking if you want to continue or not?
Sounds like your email server is using an invalid or self-signed SSL certificate rather than one issued by a trusted third-party. That is not necessarily a problem - the data would still be encrypted. But if the server usually operates with a valid certificate, there is reason to believe something is wrong. See if there's any information about it on the mail server's site, if any.