I tried all kinds of settings, POP3, IMAP, etc. All kinds of ports 995, 465, 993, etc.
It didn't matter. What worked for me, was obtaining a secure mail key from AT&T. Then, using the key as your AT&T password where it ask for your password in setting up an email account in Thunderbird.
It sure would be nice, if someone working on the Thunderbird project, would have added it in the instructions.
-The CodeSchlocker
