The settings as per your screenshot, are these the settings required by AT&T? They probably do have a support web site where you can look up that information. Please post your result here.

Yes these are the settings required by AT&T, and they were working for me last week, and they are still working for my wife's Outlook account. I really don't want to have to deal with AT&T but I may need to, but last time when Outlook wouldn't work for me anymore they were no help. I was hoping there was something on my end I could do resolve this.

Interestingly I get 404 page not found errors on the ATT web site for anything containing email settings. So my guess it they are yet again in the process of changing everything. Perhaps it has finally sunk in the the Yahoo deal was rubbish.

Interestingly enough - I went back into Outlook, set up a new email account using the exact same settings as I was using in Thunderbird, according to the AT&T recommended settings, and I can send email again through Outlook. The last time it worked for me in Outlook was in summer of 2012 - then it mysteriously did a similar thing and stopped accepting my password on the outbound side. So - does this mean AT&T change something? I have Uverse DSL. It was working using the same settings. Now it doesn't work in Thunderbird, but it works in Outlook. I guess I will go back to using Outlook. Any other suggestions as to the cause would be appreciated. Thanks all!

Did the problem start after a Thunderbird update?

Do you run any kind of anti-virus/security software with a built-in firewall?

I "believe" the problem started after my computer began the installation of the free Windows 10 upgrade without my wanting it - so I had to at least partially back that out, and that is when this started. I have re-loaded Thunderbird (I didn't uninstall it, not sure that would help?), I also tried tempararily turning off my firewall and virusscan to no avail. I have changed NOTHING on my computer compared to when it used to work in Thunderbird other than that abortive Windows10 update. So that could have done something as well.

Try to start Windows in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01 - Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Good suggestion - finally had a chance to try it today - with computer in safe mode (Windows 7) and using Thunderbird in safe mode, I still have the problem, same as before. Thanks. Maybe that tells me something?

It tells that anti-virus doesn't appear to be the culprit.

Check again with your email provider about the correct server settings and post them here, i.e. a link to a support web site (not your current settings).

Check you user name details, ATT use Yahoo. YAhyoo probably requires the whole email address to pick att customers from their own

I am asking AT&T support about this - but I am using the settings recommended by them at last published. I am using my whole email address as I always have. Very frustrating. I will keep using Outlook for now.

Matt said

Check you user name details, ATT use Yahoo. YAhyoo probably requires the whole email address to pick att customers from their own

Thanks - finally triggered that I had blank user name in my outbound server settings - it used to be filled in, I never noticed the field was blank until messing with the server settings again. If you stare at something long enough you will see it eventually. Thanks again.

Thanks to all for your suggestions/help. I did go back up and look at the screenshot above and sure enough my Username on the outbound side was blank. That must have happened during the aborted update attempt. So this is fixed. Guess it was too obvious to me.