Reverse DNS for multiple servers on VPS

rDNS can be set only by the owner of that IP(s), in other words the hosting company or datacenter. They usually offer this option with server control panel (like SolusVM) or you must ask them by opening a ticket. Just doing this in Virtualmin or directly in Bind is no good if you previously didnt set rDNS with your host (in one way or another). In case of your local ISP the rDNS cant be set as i dont know for any ISP that offers this option. Maybe it would be possible if you pay for some "premium" connection but this usually cost more than renting a server (VPS or dedicated).

Don't create a reverse DNS record for each domain. You don't need to when virtual hosting. You just need to make sure that the VPS itself is setup correctly.

If you have shell access, type "hostname -f" and see what name the server thinks it is. Do a DNS lookup on that name and see if it already points at the IP address of your VPS. You should have an A (forward) record for your VPS that points that name to the IP address of the VPS. This is something you can likely do yourself, and may already be done. This is also the name that is likely already set up on the server when it was created.

You should have a a PTR (reverse) record that points the IP address of the VPS to the name of the VPS used above. This is something your provider may already have done when you created your VPS and chose a hostname, but it is them that will need to change it if needed.

Basically, those two records should point at each other. If you lookup one, you get the other. Again, these are related to the server, not email addresses.

Then..

In your mailserver configuration, you must tell the mailserver to announce itself as the name you setup above. In postfix, this is the directive:

myhostname = myvps.domain4.com

which is in the file /etc/postfix/main.cf.

Or, in Webmin: Servers -> Postfix -> General Options -> Internet hostname of this mail system

You can do this last step yourself if you manage your own server.

If you do those three things, you will have met the requirements asked of you, as it is these steps they were referring to.

Yeah, forgot to say - rDNS is attached to the IP(s) not domains. You can have unlimited number of domains on same IP and all of them will share same rDNS.