Recommend Open Relay Filter Enterprise Edition 1.1.1

ORF Enterprise Edition is an enterprise-level anti-spam solution that helps you to block spam before it reaches your network. ORF uses DNS blacklists (DNSBL/RBL databases like MAPS) and has its own tools to stop spam on SMTP protocol level. It uses other sender checking methods like reverse DNS checking.ORF can be used with Windows 2000 built-in IIS SMTP Service and Microsoft Exchange Servers (which use this service for SMTP communication