How to properly configure loopback IP hostname in Linux?

Question

linux

Hello. There is an OS OpenSuse 13.1, which is web on a server on which there are 4 virtual hosts on which sites with different domain names lie.
There is a white IP for the server, which leads to the router, which in turn forwards to the server.
This white IP is attached to domain names. The provider has domain names registered in the loopback address.
When installing OpenSuse, it asks for the reverse IP hostname, what to enter there so that the mail works normally and it is not banned as spam?
How to properly configure multiple domains on your server? I just mostly worked with local servers or hosting, but right now it's vital to work with my server and I can't find complete information. I already made a mistake by not specifying the loopback address of my domains with the provider, and my IP was banned, since the mail outgoing from my sites was considered spam. Right now, I know this now and changed the white ip, but I'm afraid that I might have missed something else, I'm not good at networking.

Answer 1

If I understand the question correctly, then we are talking about a PTR record for a white address. With this implementation, it is more correct to do the following:
1. In the domains, register as a mail server one node from a specific
DNS domain seconadrysite:
MX mail.primarysite 10
2. In the DNS for primarysite create:
DNS primarysite:
A mail.primarysite XXXX
MX mail.primarysite 10
3. Create a PTR record for your Internet provider:
XXXX mail.primarysite
There is also an option to create your own MX and your own PTR record for each domain, but not all providers are able to support this, that's why I write "more correctly", but not "the only possible option ".