Thanks NON and Rednose for your interest and your help. I tried replacing the certificate but unfortunately, that did not fix the problem. It appears that Thunderbird goes out to the pop server and connects to the server. Thunderbird then displays a message saying that it is connected to the server. It then times out after about 10 seconds or so. I wonder if I should save the certificate from my current Avast and import it into the newer version?