Deleted my comment since you can easily view edit history. 
One note, just because some people believe that relying on obscurity is effective security, your domain name will still appear in the public certificate transparency logs every time you issue a certificate. If you’re relying on this domain not being public, you should reevaluate some design choices.
I realize it’s exceedingly likely that you are not of this mindset, but it’s worth noting just in case.
I still want to post an item about reasons why forum users would benefit by sharing their domain names, but I'm still waiting for a really useful DNS testing tool, as described in the thread
(because I wanted to create a norm that people who won't share their domain names should at least take a set of steps to eliminate likely misconfigurations as the source of their problems—but we might need slightly better tools to hepl people to do that)