Is it that you cannot send after confirming the exception? It appears from message that you have the option of allowing Thunderbird to proceed by clicking the exception option .

I did not wish to make an exception for a self-signed certificate, since my mailserver is managed by cPanel, which does automatic free and automatic certificate installation. In the past this problem did not occur.

It turns out that cPanel seems to have a new bug in it, so that at least under certain circumstances it will fail to update a free automatic certificate. Previous experience shows that cPanel does not immediately fix bugs in its products, perhaps leaving them to be fixed in a future release.

I would like to leave this question posted here, so that it can help others who run into the same cPanel bug. The solution in my case was to ask my computer operators to update the certificate manually using the management tools (probably WHM). This instantly solved the problem.