The Exchange 2003 version of Outlook Web Access is only available after you upgrade the back-end server to Exchange 2003

Microsoft Exchange Server 2003 must be installed on the front-end server that your users connect to and on the back-end server if you want your users to be able to use the Exchange 2003 version of Microsoft Outlook Web Access (OWA).

Different combinations of Exchange 2003, Microsoft Exchange 2000 Server, and Microsoft Exchange Server 5.5 determine the version of Outlook Web Access that your users can use. The following table lists the version of Outlook Web Access that users have access to based on the versions of Exchange that are installed on the front-end servers and the back-end servers:The Exchange 2003 version and the Exchange 2000 version of Outlook Web Access are substantially different from the Exchange 5.5 version of Outlook Web Access. The Exchange 5.5 version of Outlook Web Access uses ASP pages to communicate with an Exchange computer that uses Collaboration Data Objects (CDO) 1.2 and MAPI. The number of clients that can access the mailbox store at the same time is limited by the MAPI-based connection to the Exchange computer.

The Exchange 2003 version and the Exchange 2000 version of Outlook Web Access do not use MAPI to access the mailbox store, and they do not use ASP pages for client connections. Clients continue to connect to the Web Access Component through Hypertext Transfer Protocol (HTTP). However, the Internet Information Services (IIS) server that hosts the Web Access Component uses the Web Store to provide access to the user's messaging functions. IIS receives Outlook Web Access client requests as a proxy for message traffic between a Web client and an Exchange 2003 server or an Exchange 2000 server. If the server contains the Exchange 2003 database, Outlook Web Access uses a high-speed channel to access the mailbox store. If the server is a front-end server, Outlook Web Access sends the request to a back-end server by using HTTP.

For additional information about Exchange Server 2003 front-end and back-end configurations, click the following article numbers to view the articles in the Microsoft Knowledge Base: For more information about Exchange Server 2003, visit the following Microsoft Web sites: