Can not recieve Email through Firewall

We are setting up a BSD Firewall for our NT 4.0 SP6 Exchange 5.5 server. We can send email through the firewall, but cannot recieve EMAIL. When email is sent to test, we get:

Action: failed
Status: 5.0.0
Remote-MTA: dns;mail.amerivoice.com (64.109.120.121)
Diagnostic-Code: smtp;550 Relaying is prohibited

Any idea? I checked the setup twice but must be missing something.