I work for a small company and we use our own smtp and imap services on a Linux server. I am using Let's Encrypt for digital certificates on these services. Whenever I update the certificate on the server, Thunderbird shows an error on the certificate saying it is not valid and I must override the warning in order to use Thunderbird. However, the same certificate works fine with the apache web server when Firefox accesses the site. I can also use the checktls site and it reports no problems. Can you suggest a reason why Thunderbird is not accepting the certificate?