I recently switched my SSL (TLS) certificate from a regular one to Let's Encrypt. I decided to get rid of my old expiring cert files, but I physically removed them, I've started to have strange issues with Thunderbird Mail. Normally after I open the application, I get a login box. And after the old cert files removal, I stopped getting the login and also stopped receiving mail. Sending was fine. Also the SMTP server was working, since in a web mail access everything worked as usual. Only after I put back files that belonged to the old cert, Thunderbird Mail started to work as usual. I don't understand the connection between my old cert files that I was using for HTTPS and Apache and Thunderbird Mail, its login and receiving mail in it.