I have the same problem too. Thunderbird has been updating a lot lately. First my wife's email quit working then it quit working for me. We can still get emails by getting on Yahoo/ATT but can't with Thunderbird. All suggestions are welcomed.

Thanks 2281Larry

If you're using ESET you should temporarily disable SSL filtering until they solve the problem per https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products?ref=esf

The problem is being reported by dozens of users. https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/ https://forum.eset.com/discover/

Tried several times but it not make a difference. Will keep looking and trying.

I do not, to my knowledge, use ESET. Still, I did try to change TLS from 3 to 1. No change, can't receive emails. I read on the forum about getting a secure key. I read the article for obtaining a secure mail key, however, my version of Yahoo/ATT email app does not have a place to obtain a "secure mail key". I did find at sign in something about Zen Key. Has this app taken the place of a "secure mail key"? If I have missed something please let me know. I nee to start using Thunderbird again!!!!!!

how to create AT&T secure mail key: https://www.att.com/support/article/email-support/KM1240308

you may also see this page, on AT&T-Yahoo.