In a few tutorials ive read on how to make a mail server say i need a FQDN for postfix and an exact external address to my mail server for DNS records. I am hosting 4 domain names and have one server behind a router with a static external ip. I usually just forwarded ports to my server but how can i get the server.domain.com to match my setup or should i just use my external IP for the server?
Correct reverse DNS setup is only important for outgoing mail, not incoming mail. You can use as many hostnames pointing at your single static IP for users to use when submitting mail (e.g.
For sending outgoing mail you need to pick a single hostname as the canonical hostname for your mail server (this hostname doesn't have to match the domains which the server is sending mail for), for example