Exchange 2007 receive problem

I had my email host provider move my MX record of Northernpharmacy.com to point towards my firewall and we are having problems receiving email but not sending. The sender gets a error of Remote host said: 550 5.7.1 Unable to relay for blichtig@northernpharmacy.com Giving up on 63.139.89.78. Anyone have idea why the exchange server is not receiving mail correctly? Any detailed instructions on how to fix this. I thought all the bugs but when they switch the mx record it will not receive mail on the Exchange 2007 server.