Internet mail in exchange 2000

We just moved from a domino server to exchange and after setting up the new IP, DNS..etc with our ISP. Everything works fine with corp Outlook (send and receive internally and externally) as long as you are inside the network(behind the firewall). Ican use internet outlook (SMTP)to send outside of the building, but I can't receive any internet (POP3) msg. I can hit my firewall and have made sure the ports are opened. I really think it is a setting within exchange. Any clue.