Centos Web Panel server test page

My server's domain is mydomain.com and my hostname for the mail server inside of Centos Web Panel is mail.mydomain.com. The problem I have is - there's a Centos Web Panel server test page that displays at mail.mydomain.com. I want to replace that page but I don't know where it is loading from.

I've already uploaded an index.html file  in the public_html folder for mydomain.com that displays, but I can't find the location for mail.mydomain.com, where the Centos Web Panel server test page is loading from. Where is it?

My server's domain is mydomain.com ..

Falls at first hurdle: not a FQDN for a host.

You don't understand. That's not my server's real domain name. I just didn't see the importance of using my real domain name. The most important point is - where is Centos Web Panel displaying the test page from? What folder?