Can not send emails from Thunderbird

I will presume the configurations of these email clients (Outlook, T-bird, etc) are not new and probably have been successfully connecting to smtp.cox.net for years...if not a decade.

Cox had recently changed something.  PTR records are like ID cards for email servers..."Who are you"?...so it begs the question to what is Cox comparing these IP addresses?

I wonder if email clients on cell phones connected to a cellular network are also having problems.