Are you using a secure mail key?

https://support.mozilla.org/en-US/questions/1339265

Thanks for the response. To the best of my knowledge I am not using a secure mail key as I do not recall creating one. Just by chance I discovered under the "See Also" listing "server outage" on this page that I had a similar problem in 2018 and the reply was also about having a secure mail key. I had tried to create a secure mail key but was unsuccessful; however, for some unknown reason Thunderbird started working again. There has not been any problems until 3 or 4 June '21. It seems suspicious to me that this problem appears to coincide with a Thunderbird update. Do you have any other ideas?

An AT&T account might work with just the account password if the account was added to TB before the secure key policy was applied, but if the password is changed or the account is added to another computer, you will need to use a secure mail key in TB. Otherwise, if the issue occurred after a TB update, it's possible you have a security app that became incompatible. Or, it may in fact be an AT&T issue. But if you browse this forum, you will find there are many cases where a secure mail key was necessary to restore service.

Update. Was able to contact AT&T a week ago. They said they were doing some maintenance and I should be able to get email on 12 June. 14 June - still unable to receive email. Contacted AT&T again. They reported no problem on their system. Suggested re-install TB. Uninstalled TB (32 bit) and re-installed with new downloaded 64 bit. Still unable to receive email. I am still suspicious this problem is related to last (3 June) update.

have you created a secure mail key as you need to and replaced your old password in Thunderbird with it?

Update 2. I logged on to my AT&T Profile and discovered that I had in fact created a secure key in 2018 when this problem (no emails) occured in 2018. So that is undoubtedly why I was able to begin receiving emails again in 2018. I do not understand this requirement for a secure key as I continued to use my password to access my mail through Thunderbird for the last three years. At this time it appears to me that the secure key is not the problem since one does/did exist. P.S. I do not use a third party anti-virus so that cannot be a problem. I do use Malwarebytes. Additionally, I tried turning Window Defender off and running TB in Safe mode. Any more thoughts? It appears that there is no solution and I will have to find a different email reader.

I'm dealing with the same problem. Can't access AT&T/yahoo email through Thunderbird anymore and went through the same issue a few years ago. I did nothing and it was eventually corrected by Thunderbird? That an email account suddenly becomes unusable when the user altered nothing seems to put the blame on Thunderbird. Have not changed my antivirus in months, so they wouldn't suddenly become the issue. I've supported Thunderbird since they went independent, but if they can't provide seamless email, I've got to migrate to something else. And I've had Thunderbird for 15-20 years?

Update 3. Able to get TB working. What I did: (1) Deleted recently updated TB; (2) re-installed TB from backup made prior to recent (3 June 2021) update. Continued to receive server problem msg. (3) Logged on to AT&T > Profile> Manage Secure Mail Key (way at bottom of page); (4) deleted the existing Secure Key; 5) Created new Secure Key. (5) Started TB and used new Secure Key as password - Success - was again able to receive/send emails.

  In retrospect it may not have been needed to delete the recently updated version of TB.  It may have been just necessary to create a new Secure Key.  TB offered to update but I declined.  May allow later. 

P.S. Every email I select has warning msg. that TB has blocked it (images?). Need to click on Allow option. Maybe I just do not have some option set correctly.