What is the correct settings to setup a Let's Encrypt certificate for a subdomain? Running Ubuntu 20.04 / ISPConfig 3.2.1 Last night, I followed the tutorial to secure the admin (8080) of ISPConfig. https://www.howtoforge.com/tutorial/securing-ispconfig-3-with-a-free-lets-encrypt-ssl-certificate/ So now server.maindomain.tld has a proper certificate. I have setup Let's Encrypt certificates for several domain.tld. I have all of the DNS entries necessary populated, including mail.domain.tld before I setup certificates. However, now, when I setup email for the domains, using mail.domain.tld, email programs (Thunderbird and Android), claims the certificates are not valid. Pulling up the certificate info, it looks like I could use server.maindomain.tld, and that certificate does work correctly. However, I prefer not to give out the server.maindomain.tld as the email server. I do have mail.domain.tld setup correctly, and emails work fine with SSL/TLS security exceptions.