Two domains and one Linode

Generally you will have one mail server at one IP address. The reverse DNS will point to the IP address of this server. You then setup all your domains to send mail through this one mail server.

So, if you call your servers host name "server" and your 1st domain name is called example.net, you would set your mail server up to send all mail for example.net through server.example.net

In the DNS you will set an MX record to tell the word that the mail server for example.net is server.example.net

Now for reverse DNS, set it to the ip address of server.example.net

Now if you buy a 2nd domain, example.org, you modify the configuration of your mailserver server.example.net to also handle mail for the domain example.org

Now in the DNS, set another MX record to tell the word that the mail server for example.org is server.example.net

The reverse DNS does not change.

Ah ha…. this all makes sense now. A very clear and concise answer. Thank you very much I've been trying to figure this out for quite sometime and you just answered it in a few paragraphs.

Cheers! :D