Apart from securing your servers so that spam generators can't use them as relays, you want to protect users from all those annoying messages. Organizations such as the Mail Abuse Prevention System (MAPS) build realtime block lists to help dispose of unsolicited commercial email (UCE), essentially diverting such messages to a null network device so that the messages never appear on your servers. UNIX and Linux servers have traditionally incorporated programs such as sendmail to support block lists.
Now, Microsoft Exchange Server 2003 builds on Exchange 2000 Server's SMTP capabilities to offer built-in spam-blocking support. Exchange 2003 lets you configure block lists as connection filter rules for your organization. To configure these rules, open the Exchange System Manager (ESM) console, select the Global Settings node, and open the node's Message Delivery Properties dialog box. After you configure the rules, you put them into effect by applying them to each SMTP virtual server that handles external traffic. For each rule, you specify the list provider, the provider's DNS suffix (so that Exchange can maintain an updated version of the list), and the priority you want to give the list. Some lists are better than others at blocking various types of UCE, so you can use multiple rules, each specifying a different list, to attain a desired degree of protection. Figure A, for example, shows a rule that implements a general block backed up by a rule that blocks what you might call "bad habits." Exchange 2003 also lets you configure lists of known individuals (identified by email addresses) from whom you don't want users to accept email.
This new antispam feature makes Exchange 2003 a viable component at any point in a messaging infrastructure, not just behind bastion servers running UNIX or Linux. Smaller companies that can't afford to deploy multiple layers of servers to defend their network will especially appreciate this addition. Furthermore, Microsoft Office Outlook 2003 and Exchange 2003's version of Microsoft Outlook Web Access (OWA) include client-side junk-mail filtering, so you can impose blocks on both the client and the server.