PoP 3 and exchange 200 server

I am using exchange 2000 sever on win 2000 server.I have setup a POP3 server for users to retrieve mail from the their home via the internet. users are able to retrieve mail but sending mail using same credentials is a problem. (error 550 7 unable to relay)the solution provived by microsoft did not work and i did set the server to relay authenticated users regardless of domain. any help would be appreciated.