Hi, One of my users (who uses the same domain like me for E-Mail, just another E-Mail-Adress) got a certificate problem: Thunderbird reports that "this website tries to identify itself with invalid information" (or similar, translated from German). The user is meant to issue an exception, but the warning pops up again and again. The latter may be an issue of Thunderbird, but the error message itself hints at a certificate problem. The interesting thing is that he hasn't done any changes, maybe except for updating Thunderbird. For two years the mailing has worked flawlessly. What I find even more interesting: when opening the certificate information, it says the the certificate was issued by Bitdefender. I'm pretty sure it should say "Let's Encrypt", because the certificates were issued with certbot. On the other side, issue date is correct. I don't know where to look for the fingerprint, else I could check that as well. The server runs CentOS 7.7.1908
Check with your user that he really used the hostname of your mail server as smtp and pop3 server in thunderbird and not a subdomain of its own domain. Some antivirus software on windows puts itself as a proxy between thunderbird and the mail system, that's probably why Thunderbird sees a different cert than your server cert. Might also be that this is the whole cause for the error in Thunderbird and not a problem with your server cert or a wrong domain name used by the customer.
