Configuring SMTP for Both Authenticated and Anonymous Connections at Microsoft
http://www.microsoft.com/downloads/details.aspx?familyid=b1e05053-3e12-4603-89d4-9c5e5d83cd46&displaylang=en
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.
