550 5.7.1 Unable to relay for smtp server

Open the Exchange System Manager;

Go in Administrative Groups -> Administrative group name -> Server -> Server name -> Protocols -> SMTP;

Right click on Default SMTP Virtual Server -> properties;

Access tab -> Relay buttom;

Select "only the list bellow" and add your domain and\or IPs you do want to allow the relay

and finally check the "Allow all computers witch successfully authenticate to relay, regardless of the list above" checkbox.