Configuring SMTP for Both Authenticated and Anonymous Connections
"To reduce administrative overhead, the Microsoft Information Technology (Microsoft IT) group needed to change the way Microsoft Exchange Server 2003 makes internal Simple Mail Transfer Protocol (SMTP) connections. Microsoft IT set the SmtpIpRestrictionFlag attribute to 1 to configure an SMTP virtual server to allow anonymous connections from an explicit list of IP addresses, yet allow all authenticated connections from other Exchange servers."
Read the complete story: Configuring SMTP for Both Authenticated and Anonymous Connections at Microsoft