They suggested I call Yahoo since AT&T actually uses Yahoo to handle their email infrastructure but such a call is a virtual impossibility to any bloated outfit like Yahoo, Gmail, etc. However, I talked to AT&T tech support and they said they didn't have the certificates and didn't know wh ere I could get one.
I saw a posting on another site in which the poster said someone from TheBat! support told him to get the correct certificate from the server administrator and install it manually. Where are Outlook and my other client application getting the certificates they successfully use for these servers, where are they being stored, and is it possible for me to access them and try them with TheBat! ? I looked in Outlook to see if there was some way to export certificates but I couldn't find anything that even listed them. I then tried the same server in Outlook 2010 as well as another email client I use and it all worked perfectly! So, it seems to me that either TheBat! is using the wrong certificate or is interpreting its contents incorrectly. The name of the POP3 certificate it loads is .com and for IMAP it is *. In a Hail Mary effort I deleted the two certificates TheBat! was using for these servers fr om it's Trusted Root CA address book in hopes that the correct ones would be reloaded when I accessed the mail servers, but it just reloaded the same ones again and the problem remained. The server host name ("") does not match the certificate." The server host name ("") does not match the certificate." When I change to IMAP and use incoming email server I get the similar message "TLS handshake failure. If I instead set it to as AT&T currently recommends I get the message "TLS handshake failure. If I specify the incoming mail server as .com or (which AT&T used to suggest and which used to work) I get the message "FETCH: Connection to host broken (last commands sent were: "PASS", "STAT"). However, the POP3 connection to my att.net account stopped working some time ago, although the SMTP connection still works fine.
I've been using TheBat! for years and am currently using v8.2.4 64-bit on Windows 10.