First, Download & install the latest version of Thunderbird, which is 52.2.0.

Then if there is still a problem, check to make sure that your antivirus/firewall program is not blocking Thunderbird.

The #1 reason for Thunderbird to not be able to send or receive e-mail right after the program has updated, is that the antivirus/firewall software installed doesn't recognize the new Thunderbird version, so blocks it.

The situation usually resolves itself when the antivirus/firewall software updates in a day or two.

Or you can go into the settings of the security software to tell it to allow Thunderbird.

Please reply here after doing these two things. If there is still a problem, we can go from there.

Thank you,

I resolved the problem over weekend and it was not Thunderbird related. Rather an coincidence.

To temporary "fix" the issue, I set up Thunderbird to use insecure connection and send password plain. It would always fail when SSL was in use.

At about time I upgraded Thunderbird, mail server had its SSL certificate screwed up. Took me a while with cPanel support to figure out the problem and re-issue new certificate.

The problem with Thunderbird, however, is the fact it did not tell me about it. Thunderbird was just sitting there, showing "Checking mail server capabilities" forever. It should show some sort of error popup saying certificate of server is no longer valid.

Thanks, Rudolf

It might be useful to point out this issue with Thunderbird by submitting a bug.