Mailbox unavailable for IIS 6 smtp service

     After created a smtp virtual server and enable the smtp service, if the web application still got error such “Mailbox unavailable. The server response was: 5.7.1 Unable to relay for name@email.com”, need to do following:

1. Open “IIS 6 Manager”
2. Under local computer name, find SMTP virtual server, and open properties for it
3. On “Access” tab, click “Relay” on “Relay restrictions” section
4. Choose “Only the list below”, then add loopback ip “127.0.0.1” and server ip to the list
5. Restart smtp service