Mail cannot verify my mail server's root certificate

When I try to send an email, I get a warning message saying that mail cannot verify my mail servers root certificate. I have tested the certificate and it had a blue boarder not a gold one, I can't remember what this means but I think its not recognised. This has only just started happening.
Apparently the root server is VeriSign Class 3 Extended Validation

Mail was unable to verify the identity of this server, which has a certificate issued to "smtp.mac.com". The error was:

The root certificate for this server could not be verified.

You might be connecting to a computer that is pretending to be "smtp.mac.com", and putting your confidential information at risk. Would you like to continue anyway?