I found the answer, finally--posting here for anyone having the same problem with att/yahoo mail. This does not happen consistently, but if your password is rejected and everything else appears to be in order, go to your att profile page (NOT Yahoo), click sign-in info, then "secure mail key" to create a mail key. You will be shown a random string of letters which you should copy and paste into the password field when setting up or re-authorizing your account in TB--paste in both places, for incoming and outgoing servers. So it was not a TB problem! TB happened to be the app that was incompatible--this time. As I said, it does not happen consistently. I have reset my password at Yahoo before without encountering this problem. I have to give credit where due: I had downloaded Mailbird when I thought I would not be able to recover the account on TB, and it was a Mailbird help doc which finally gave me the clue that led to the solution.