Root certificates are handled by the OS automatically. Looking at your screen shots however I thin the host names are out of date. They should be pop.cox.net and smtp.cox.net. I'd suggest updating those and see if the certificate error clears.
All settings are updated per above. Sending-receiving OK. No problems until I try to send to multiple recipients. Error messages say "Unable to establish a secure connection to the server because the correct root certificate is not installed" and then errors to say " internal error". Only started having issues since today.
thank you. that seems to be the issue. i get that they are trying to deter spam mailings but for those of us who are in a large philanthropy organization (or any large group) can't function this way. will check out link. thank you!