well I checked the sites encryption sertificate https://www.immuniweb.com/ssl/pop.scdconline.org/MyCiE80S/

as the server name you are using is https://www.immuniweb.com/ssl/pop.scdconline.org/MyCiE80S/

as the server you are using is pop.scdconline.org and the certificate returned in the test was for *.hostingplatform.com I am guessing someone is operating a minimum cost server without all of the relevant certificates. Perhaps you need to add an exception for that server if you trust the server is actually correct despite the name not even looking vaguely similar. Or perhaps you need to change your connection security to none and not use an encrypted connection.

Thanks Matt. I'll try your suggestions. I still don't understand why upgrading Thunderbird apparently triggered the problem. Roger

I still don't understand why upgrading Thunderbird apparently triggered the problem. Roger

I don't think any of us really know about the encryption certificate layer in detail. All I know really is folks that know a lot more than me about it are forever tinkering there to stop known exploits and that tinkering inevitably leads to unexpected outcomes in some use cases. Those that have email scanning enabled being the most affected because the folk that write antivirus products try and decrypt the data in transit using questionable techniques very much the same as man-in-the-middle hacking.

This "layer" is part of the shared Mozilla platform that both Firefox and Thunderbird share but is developed by the Mozilla company along with Firefox. While the folk involved are very good at what they do, user interface, and user notifications that make sense are not their strong point. This also means that it is developed and maintained in isolation from the rest of Thunderbird development. Essentially the developers involved do not even work for the same company. This does result in changes that at times really only get "found" when something breaks. It is not ideal, but if that development was also on Thunderbird's (Mzla corporation) plate it would in all likelihood have stalled a decade ago due to lack of resources.

Over the years there have been numerous bugs where changes in the certificate store and processing have resulted in certificate exceptions being either ignored or nullified without anything to give a clue there was a problem. I even recall one where clicking the create an exception simply silently did nothing. Because it involves antivirus (with its attendant VPN and software firewall) and the semi external basis of the certificate code, it is a first place to look when an update does something odd and mail just stops.

A quick look at these patched vulnerabilities shows lots more going on under the hood than release notes really indicate. They just link to this list in the hope no one looks I think. https://www.mozilla.org/en-US/security/known-vulnerabilities/thunderbird/