... 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 name of the POP3 certificate it loads is legacy.pop.mail.yahoo.com and for IMAP it is *imap.mail.yahoo.com. I then tried the same inbound.att.net server in Outlook 2010 as well as another email client I use and it all worked perfectly!  So, it seems to me that either ...