Being an At&T customer, I made the mistake of associating my login email address be an equivalent to my phone number as an At&T login userid. My email address is blahblah@sbcglobal.net, and has been, for 19+ years. This association, unbeknownst to me, also caused immediate failure of TBird ability to login for email. The fix is to use TBird tools security passwords to delete all saved passwords for the afflicted email address, and then in AT&T Profile, locate email tab, scroll down to secure mail key, provide a token, like KDYouFool and then the dialog creates a secure mail key. Copy the key to clipboard, go to TBird, click check mail, and paste the provided key into the password bix. This corrected both POP3 and IMAP setups, and the same secure mail key is used for all other instances of mail, for that email account, don’t get a different key... Hopes this saves someone some frustration, and if you find you can login to yahoo mail, but not get the mail with TBird, as it just sits there doing nothing, finally erroring out, then thus likely will fix your problem. Note that nobody at AT&T knows how to fix this, and many of their online screens crash, as they are rather incompetent programmers and/or customer service reps.. They will only reply that it is WEIRD that you can yahoo mail, but not TBird or iPad mail.