Julia, I'm having a similar problem which I'll describe below. I would also appreciate your help. Thanks.
I am a non-technical guy who would appreciate any help on this.
We have 3 laptops in our home, 2 running Thunderbird and 1 running Windows Live Mail. The latest TB version is installed: 78.3.2. Our ISP is Spectrum. The last several months everything has been running fine but beginning yesterday (10/7/20), on the Thunderbird computers we started getting the message "Connection to server mail.twc.com was reset". On the Windows Live Mail computer the message was "Unable to send or receive messages for the pop-server.austin.rr.com account" (etc.) and we (mostly) can't send or receive emails. I say mostly because once or twice since the problem began yesterday, we would get a dump of several emails, then it would go right back to the error message.
Our internet connection is fine and we are fully functional on the web. We have not fiddled with any of our email settings (ports, etc.). And I can access our emails through Spectrum's webmail (although I dislike using it and much prefer TB).
I have read of a few possible fixes as to TB and tried one: changing the TLS Security Version minimum from 1.3 back to 1.1. It didn't help so I switched it back to 1.3, the current default.
I think I saw that Thunderbird ver. 78.3.2 was automatically installed a couple of days ago, so thought maybe that was it. But that wouldn't explain why Windows Live Mail began having the same problem at just the same time.
I called Spectrum Tech. Support, and they were no help, insisting that since I could send and receive email via their webmail, it was a "syncing" problem and they couldn't do anything. I just don't buy it---two different email programs having the same problem, beginning at the same time. It seems like the problem must be with the Spectrum servers.
I would really appreciate your help getting this resolved.
To get started, are you able to access email at Spectrum webmail?