Getting a mail server to work across the internet

Hi there, i am quite new to all this so its probably just me being a newb 🙂 just wondering how to go about getting my local mail server to send and recieve mail from all other mail servers on the net??….

I am using the pop3 server in windows 2003 i have an active directory setup, and i have registered my domain on the net with telivo, i have also setup dns with freedns.afraid.org and changed the name servers of my domain to look at freedns.afraid.org. just cant seem to get it to point at my mail server so that i can send / recieve mail… would really apreciate it if someone could take me through getting this to work, (if its possible) thanks in advance… chris