I suggest you contact Eternal September about their SSLTLS certificate. While is may just as well be your anti virus program messing about it would be good to start with a known valid certificate being issued by the server.

Thanks. I will hit Eternal September up and ask others in the News Group. My Mac does not have an anti-virus program although I recently installed Malwarebytes. I will try turning that off, too.

Thanks for the tip. It was Malwarebytes that caused the problem. When I uninstalled it, Thunderbird started working again. Rather annoying that I got the warning without any indication of where it came from.

LexingtonVAPin said

Thanks for the tip. It was Malwarebytes that caused the problem. When I uninstalled it, Thunderbird started working again. Rather annoying that I got the warning without any indication of where it came from.

When your car engine light comes on you get an error, usually without any other information. Unless you see steam coming from the engine bay or a stream of oil following you down the road. Likewise in this instance an error situation was arising where Thunderbird knew the certificate it was getting was expired. The exact cause was unknown as the only information received in a certificate transfer is the certificate. It is just a shame so called security software so frequently creates a mess and is so deeply integrated into the operating system that it is not even obvious it is involved.

By it's nature of monitoring encrypted communication it must be decrypting those communications to determine what is in them. ipso facto it is busy hacking your communications, to protect you from hackers. Hacking rarely ends well.

Interesting analogy. With car engines, an error code leads to the cause. One just needs the interface to read the code.

It was not clear to me that the cause was Thunderbird, considering that the issue may have started when I installed the malware scanner and definitely ended when I uninstalled it.

Accurate or not, I do not consider this to be a Thunderbird issue, but an issue caused by another program, Malwarebytes.