Windows Server 2003 provides significant enhanced functionality that Exchange 2003 takes advantage of:

Outlook HTTP access:

IIS 6.0 and Windows RPC Proxy service in Windows Server 2003 enable communication between Outlook 2003 and Exchange Server 2003 by means of HTTP. Outlook 2003 users can synchronize directly with the server running Exchange Server 2003 over a HTTP or HTTPS connection.

Internet protocol support:

IIS 6.0 provides Exchange with its support for many common Internet access protocols that increase the flexibility of the operating system, such as HTTP, Post Office Protocol version 3 (POP3), Internet Message Access Protocol version 4 (IMAP4), and Simple Mail Transfer Protocol (SMTP).

Active Directory:

Windows provides Active Directory, upon which Exchange depends for user information, mail routing information, user authentication, and LDAP read and write functions.