How to create a firewall rule / packet filter rule

Sorry for asking such a beginner question.
I need to know the basics of creating a rule.

I need to let my mail server talk to the DCC Servers through a specific port.
By doing this I am assuming that this will allow them to talk back and forth.

Source = (External (Address)
Service = (DCC Servers)
Destination = (Mail Server)

Is this correct?

Also when creating my Mail Server. Is it better to create it by way of
Host = IP Address
or
DNS Host = mail.Server.com