Please see https://support.mozilla.org/en-US/kb/cannot-receive-messages

already tried these. didn't work. thanks

The correct POP server for AT&T is inbound.att.net on port 995, SSL/TLS, and a secure mail key must be entered in place of the account password.

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

See https://support.mozilla.org/en-US/questions/1313640

did not solve the problem. still get the server error

Did you remove the passwords from Saved Passwords in Options/Privacy & Security, then restart TB and enter the mail key when prompted?

yes i did that.

Do you have any kind of security app scanning the TB profile or secure connections? Some, like Avast, block connections from TB.

i don't believe i do. the problem started 9 days ago & none of my settings had changed & i've been using TB for many years

I'm currently having this same problem. I've done the setting check, delete password, started in safe mode and nothing is helping. I can get to my emails via yahoo and firefox and att web site but not thunderbird. happening the same on two different computers that i have also.

What's going on?

I'm also having trouble. I was primarily using windows live mail on my main computer, and thunderbird 78.5.something on my tablet. I can log into my account using a web browser, and I can use the new default windows program to get mail, but I hate it, and don't want to touch it ever again. It seems strange to me that both Live Mail and Thunderbird would be shut down at the same time on 2 different computers.

Oh, and I tried changing that security.tsl setting from 3 to 1, and that didn't help. (It also didn't help that the instructions didn't point me toward where those controls are actually located.)

So, I have to conclude that either AT&T is under attack (and has been for over 2 weeks) and not advertising, or they've implemented some new "feature" that blocks out some of us and haven't communicated that to Mozilla for update/patching.

Open to suggestions.

Many users have experienced problems with AT&T recently due to having incorrect settings or not using a secure mail key:

Enter the recommended settings, then create a secure mail key and use it in place of the account password, with normal password authentication.

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

https://support.mozilla.org/en-US/questions/1313640

https://support.mozilla.org/en-US/questions/1314254

the solution is at https://www.att.com/support/article/email-support/KM1240308. it worked, finally

I have the same problem. With comcast, google,and compuserve accounts. I deleted the passwords and changed the TLS settings. It does not even ask me for the passwords on the accounts I deleted the passwords for. There is clearly something going on in thunderbird. I even updated to 78.5.1.

I might have fixed the problem everyone. Do the following below but DO NOT change the Authorization method in Thunderbird account setting. Keep it at normal.

Create a secure mail key Learn how to create a secure mail key from your mobile device, tablet, or computer. Have your User ID and password ready to sign in to myAT&T.

   Go to Profile > Sign-in info.
   Select the email account that you want to get a secure mail key for. (You’ll find a drop-down menu at the top if you have multiple accounts.) 
   Scroll to Secure mail key and select Manage secure mail key.
   If you have more than one email address, select the one you want to use.
   Select Add secure mail key.
   Enter a nickname for the secure mail key to make it easier to recognize.
   Select Create secure mail key.
   Select Copy secure mail key to clipboard. (Jot down your secure mail key, so you have it handy if you have to update an email app on several devices.)
       For security purposes, the secure mail key only shows until you select OK. 
       If you lose or forget the secure mail key, you can create new secure mail keys as needed.
   Select OK.
   Go to your preferred email app and replace the existing password with your secure mail key. (For an IMAP account, delete the existing password for both the IMAP and SMTP servers and replace them with your secure mail key.)

I did this and it's working so far on one computer. trying the other later

So I use thunderbird and nod32. I turned nod32 off because there was a reddit thread about this. I have also done everything above. Sending is not a problem so server connection seems fine. Connection to download e-mail still does not work. Is there a log that can show any details of connection?

So I use thunderbird and nod32. I turned nod32 off because there was a reddit thread about this. I have also done everything above. Sending is not a problem so server connection seems fine. Connection to download e-mail still does not work. Is there a log that can show any details of connection?

If you have done the above items that have worked for others, and https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products?ref=esf, and still have trouble, please file a new support request https://support.mozilla.org/en-US/questions/new/thunderbird/form

I started in safe mode and had no problem. I'm going with nod32 as the problem. Much harder to turn off than I thought.

Turning off is ESET is not a solid solution. You will want to use the instructions in the link.