manual

Getting Started Guide Installation for Microsoft Exchange 2003  19
Screenshot 12 - Relay options
5. Select the Only the list below option and click Add.
6. Specify IP(s) of the mail server(s) that are allowed to route emails
through your mail relay server. You can specify:
 Single computer - i.e. Authorize one specific machine to relay
email through this server. Use the DNS Lookup button to lookup
an IP address for a specific host.
 Group of computers - i.e. Authorize specific computer(s) to relay
emails through this server.
 Domain - Allow all computers in a specific domain to relay emails
through this server.
NOTE: The Domain option adds a processing overhead that can
degrade SMTP service performance. This is due to the reverse DNS
lookup processes triggered on all IP addresses (within that domain)
that try to route emails through this relay server.