Reverse DNS entries can't always be created

Issue description

The following (from your README) is not possible on many residenial ISPs:

Set rDNS (reverse DNS) entry for server Wherever you have rented your server, you should be able to set reverse DNS entries for the IP’s you own. Add an entry resolving 1.2.3.4 to mail.example.com

What I expected to happen:

I expected to see a small discussion of things that are still possible without having such a reverse DNS entry and I expect the incorrect information to be changed such that it tells a correct story.

What happened:

I did see false inaccurate information, because I even own this server, but there is no way to get a reverse DNS entry, because the ISP refuses to create one.