Your issue appears to be that your Verisign 3 root SSL is either corrupt or missing, which invalidates your certificate. One thing to check is Open your Keychain and look at the System Roots and verify that you have the "VeriSign Class 3 Public Primary Certification Authority - G5" Certificate. If not you might want to try going to Verisign directly and downloading and installing the certificate.
You can download it here:
https://knowledge.verisign.com/suppo...=1252934277957
I am not having issues with the Certificate no longer being valid, for some reason Mail.app no longer want's to use the certificate when sending. Anyone seen this issue.