Exchange Server 2003 with SP-2 includes the Intelligent Message Filter. You can enable a number of settings in there to help block spam. For example, you can add real-time black lists, you can add specific email addresses or domains to block or you can configure a generally smart filter to block messages based on a score rating. My personal preference is to move messages with a score of 4 to the users’ Junk EMail folder in Outlook, and a score of 6 or higher to outright reject (bounce). See the screen shot at http://brad.dinerman.com/techtips/TechTarget/IMF.gif for an example.
If you want even more functionality than Exchange natively provides, then you can purchase a third-party spam filter, whether it’s software-based and which integrates directly into Exchange, or whether it’s hardware-based and sits between Exchange Server and your fireall.
Examples of filters include:
GFI Mail Essentials (http://www.gfi.com/mes)
Sunbelt Software Ninja Email Security (http://www.sunbelt-software.com/Business/Ninja-Email-Security)
Postini (http:www.postini.com) – This is actually a hosted antispam service. It does not reside on your network.