Exchange Server 2003 runs better on Windows Server 2003 than on Windows 2000 Server. One reason is that Microsoft Internet Information Services (IIS) 6.0, which is built into Windows 2003, includes several architectural improvements from which Exchange 2003 directly benefits. In general, IIS 6.0 provides improvements in management (i.e., simpler interfaces and easier navigation to required items), performance, scalability, and security.

The IIS 6.0 architecture builds on a more robust model that employs a kernel-mode listener (http.sys) and the WWW Service Administration and Monitoring component, which is essentially a user-mode process manager. IIS 6.0 effectively implements Web applications—whether they're third-party applications or a Microsoft application (such as Exchange 2003) that uses IIS 6.0—as worker processes. This model is robust because the worker processes operate independently of one another and thus have little effect on one another's reliability or security. Working with IIS 6.0's Worker Process Isolation Mode (WPIM), the WWW Service Administration and Monitoring component can detect a failed application process and automatically take corrective action—typically restarting the process. However, this combination also ensures that the W3Svc service needs to be restarted on fewer occasions.

IIS 6.0's new design also improves overall server reliability because http.sys, rather than an arbitrary Web application, conducts all kernel-mode access. IIS 6.0 is thus better positioned to deal with any exceptions that result from illegal or failed operations.