Hi
Welcome to the forums.
If you're behind a NAT router and you have a public IP address with MX records hosted outside your LAN you need a local DNS server and a split-DNS config. Have a look in the wiki, there's a description of how to set it up in there.
You'll also need to the the 'use DNS lookups' checked on the admin gui/Global Settings/MTA tab. This is all assuming that you want your server accessible from the internet (Port 25 should also be forwarded through the NAT router) rather than just relaying through an ISP mail server.