Are you using a 'secure mail key', or app password in TB?

https://www.att.com/esupport/article.html#!/email-support/KM1240462

I wasn't sure what these terms meant until I read the linked article, which helped me to gain some comprehension. Given that the term "app" is nearly always applied to items downloaded to a mobile device (e.g., a phone), and I never have used anything but a computer to handle email, I think I would answer "no" to those two questions. To be clear: Everything I said in my question had to do with a computer.

I just now realized that my inbound mail was not coming through, either. I checked on the ATT/Yahoo server and saw several emails. This may have happened in concert with the outbound failure. I tried "Get Messages" with Thunderbird and, after maybe a minute of waiting, this message appeared:

  Error with account [username]@sbcglobal.net

  Sending of password for user [username]@sbcglobal.net did not
  succeed. Mail server att.inbound.net responded: Server error—please
  try again later.

I never have experience this inbound failure before.

If you're using TB to access ATT/Yahoo mail, you have to create a secure mail key and use that instead of the normal account password. TB does not at present support OAuth authentication for Yahoo.

So, I guess "app" can refer to Thunderbird employed on a computer. Is this something AT&T just implemented, do you know? AT&T never communicated anything about this to me in any reasonable (or unreasonable) manner, and given that they collect my monthly payment, they should have done it.

As someone who has had AT&T email for quite awhile---some years ago changed to Yahoo's management, now Oath/Verizon---I find it distressing that they would assume their customers should discover these sorts of disruptive things through osmosis or by reading tea leaves.

Thank you for your help. I will carefully study that article you linked, which should clarify what I need to do for Thunderbird functionality to be restored. If problems continue, I'll ask a separate question about it.

sfhowes: First, thank you very much for the assistance. Email through Thunderbird now works properly. Second, although you may be acquainted with the unfortunate customer "care" AT&T supplies its email customers, this little summary of my adventure with them may be instructive as you work with others suffering under ATT/Yahoo email.

I used that linked article you supplied, but it was not clear why a person could not simply use a long, complex alphanumeric/upper-lower password instead, which I do, with a different one for each account or purpose, organized on a flash drive kept separate and secure away from my computer.

So, I engaged a tech using AT&T's "chat." He escalated it to some specialist, who, after I described my experience and asked my question, said that probably I could use an existing complex password instead of one of their secure mail keys (it turns out that this is quite wrong, given that AT&T generates and applies the key, not the user).

I told this "tech" what you had said; he asserted that the fault was all with Thunderbird, and proceeded to list three or four probable causes for the failed email issue, none of them having anything to do with passwords. He said he could solve this easily simply by performing a remote study. I was not comfortable with that, but he insisted that it would take care of the problem. Meanwhile, all discussion of OAuth and secure mail keys suddenly disappeared.

Then he told me that all I had to do was to buy a service agreement, and he would get right to work. I invited him to place the agreement in an area devoid of sunlight, and discontinued the chat.

I decided to follow the directions for creating the secure mail key and leave off being perplexed over why it was necessary if it was to replace an even more complex password. The procedure worked just fine; I replaced the old password, used both for Mailbox and SMTP in the "Passwords" section of "Security" in Thunderbird. After rebooting Thunderbird, I discovered that mail now works just as it did eight hours ago, before all of this started.

To think that I could have paid $49.00 for this guy to invade my computer and then not solve the problem, very likely making things even worse.

Just one more evidence of the unfortunate decline of the once-storied AT&T.

Thanks again. I will mark this "solved."

ridiculous! I am not at all someone with a degree in computer science! All I w ant to do is delete the whole email address and its existence and start over or not! THunderbird SHOULD respond to SIMPLY !!!!!! tell me HOW to do that!!!