Unverified Certificates

I'm frequently receiving this popup: "Mail can't verify the identity of the server XYZ. The certificate for this server is invalid. You might be connecting to a server that is pretending to be XYZ which could put your confidential information at risk."

When I look at Keychain Certificates I see only one unexpired self-signed root certificate; it has not been verified by a third party.

Double clicking on it shows the same long string of "random" alphanumerics and dashes for both the Organization and the Common Name for both Subject Name and Issuer Name. The Serial Number, Version and Signature Algorithm are all specified.

How can I determine whether or not to delete this certificate?