I have (2) ATT Yahoo emails accounts. Both have the same POP / SMTP setting. One of the accounts recently started getting ATT Server error messages..."Sending of password for user mkal1@sbcglobal.net did not succeed. Mail server inbound.att.net responded: Server error - Please try again later". I can log onto this particular account from Firefox, my iphone, android tablet. Only issue is from Thunderbird. I went as far as removing the stored PW, shut down TB, and restart, re-enter my PW, same error. I even removed the problem email (did not check the box for delete the data/emails), and created a new profile using the know working id/pw, same error. I then changed the log PW on AT&T...same problem with TB....iphone and android log on correctly I'm almost ready to go back to outlook...much more stable and easier to use.