Are you using a secure mail key in TB in place of the account password?

https://www.att.com/support/article/dsl-high-speed/KM1010523/

Thank you SF,

I just used my password and didn't even see where I might use a key instead.

Remove the password from Options/Privacy & Security, Saved Passwords, restart TB, enter the key when prompted for a password.

Yep, that was the trick! Big thanks SF!!

It's downloading 15k emails but thousands are duplicates, any app/program to delete those?

Try this add-on: https://addons.thunderbird.net/en-US/thunderbird/addon/removedupes/

Thanks again SF! I will give that dupes add on a try!

This is probably simple, maybe searchable but what is this error for outbound?:

"Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) outbound.att.net was lost in the middle of the transaction. Try again."

First message I've tried to send, a couple times. ~Kirch

When you removed passwords from Saved Passwords, did you remove the one for the outgoing server, the one that is listed with smtp://? It should use the same secure key as used by the incoming server.

Yes, the only saved password is for inbound, how do I set the key for outbound?

Thanks again, ~Kirch

The first time you send through the outbound server, you'll be asked to enter a password. Enter the secure mail key and check the box to have it stored in Saved Passwords.

I restarted the program but this dialog box doesn't come up. Is there another way to enter the key for the outbound? I'm really not a noob, I just act like one!

Restarting alone will prompt for the incoming password, but for the outgoing you need to send a message.

This is the only dialog I receive when I send a message, from there where do I go? This seems a bit different than a password fail response. Thanks in advance and for all your time here SF! ~Kirch:

"The message could not be sent because the connection to Outgoing server (SMTP) outbound.att.net was lost in the middle of the transaction. Try again."

Do you have outbound.att.net on 465, SSL/TLS, authentication = normal password? What is the antivirus and its settings for scanning mail?

That was it, the ssl/tls outbound, thanks!! My TBird is running great!! ~Kirch

Didn't see where to start a new dialog for TB. I now have this error:

The DELE command did not succeed. Error marking a message as deleted. Mail server inbound.att.net responded: Too many requests.

I found it in the archives 2019 but it doesn't solve the problem, has this been figured out? Or is this with ATT-Yahoo?

Thanks! ~Kirch

As with the outgoing server, is your incoming inbound.att.net on 995, SSL/TLS security, authentication = normal password?

What is the interval between checks for new mail? There are many reports of this error message. One suggestion is to rebuild the Inbox and Trash folders:

http://forums.mozillazine.org/viewtopic.php?p=14779421#p14779421

Have not been able to use TB for inbound email for a few months, outbound is fine, just doesn't get messages, times out. Set up is correct, as above. Thanks for any ideas.

What is the antivirus, and what are its settings for email?

Set up exactly as your setting show just above. McAfee but it's off since I haven't renewed yet.

Antivirus settings are not the same as mail server settings. I recommend you completely remove McAfee and use the built-in Windows Defender, and add the TB profile folder as an Exclusion in WD settings.