Having problems with email clients not recognising certs and Nextcloud message about unable to identify fqdn. Both seem to be about specifying ServerName globally? Is that correct and is there a preferred way to do this in ISPCONFIG?
The server name is the system hostname that you set before you install ISPConfig on your server. The system's hostname is configured in the files /etc/hostname and /etc/hosts of your Linux system. If you are using Debian or Ubuntu, then additionally also in the file /etc/mailname. All SSL certs that are created during ISPConfig installation are based on the information that you have set up in these files. Regarding email clients, the most common mistake is using the wrong details for smtp and pop3/imap system in the email client. You must use the system's hostname there and not any subdomain of an email domain. See this tutorial on how to configure your mail client correctly: https://www.howtoforge.com/ispconfig-email-account/
