I have noticed in Virtualmin that for all my hosts is created automatically a subdomain called mail.domain.com and is loading the default apache web page as per attached.
There is any way that I can rid of it or at least block it through the .htaccess file
Thanks in advance
That domain is typically used for accessing the system in a mail client – it’s not designed for displaying a website.
Since Apache isn’t configured with that name, it shows the default Apache site.
What you could do though is change the default Apache website, so that the site that comes up is one you don’t mind folks seeing… that’s something you can change by going into Server Configuration -> Website Options, and there you can set a domain as the default for the IP address.
Alternatively, you could setup a .htaccess file within the default domain, and using it, block or redirect attempts to browse to certain domains.