I have been using Thunderbird for email for many years. On or about Oct-20-2020, I installed the latest update to Catalina 10.15.7 Around the same time, Thunderbird 78.3.4 stopped receiving emails on two different accounts. If I restarted my iMac, for a few tries, Thunderbird would receive email on one of the accounts. I then tried reloading MacOS via Command-R recovery service. I was then able for a few tries to download emails from both service providers (ptd.net comcast.net). Then it stopped reading any new emails. Restarting did not help either account. Newer Thunderbird 78.4.0 does the same thing. This all seemed to start with my initial upgrade to Catalina. The install went badly which I blame on the ESET virus protection screwing with the ports. The initial install of Catalina was incredibly slow with ESET installed. After I backed out some of the features it got better. When I finally removed it completely, and reloaded MacOS, it finally worked about as well as before. Every install of a Catalina update after that also went badly, requiring at least a second attempt to get it in, often requiring a Disk First-Aid. I tried the reloading MacOS via Command-R recovery service again, but now the email reports "connection refused" all of the time. Since it did work a couple of times previously, I believe the server values are correct.