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You can only use Microsoft Outlook Web Access (OWA) in Microsoft Exchange 2000 Server or in Microsoft Exchange Server 2003 to access Exchange 2000 mailboxes or Exchange 2003 mailboxes. You cannot use OWA in Exchange 2000 or in Exchange 2003 to access Microsoft Exchange Server 5.5 mailboxes because OWA in Exchange 2000 and in Exchange 2003 is tightly integrated with Exchange 2000 Server. You can use Exchange Server 5.5 OWA with Exchange Server 5.5 mailboxes, and Exchange 2000 mailboxes.
OWA in Exchange 2000 and in Exchange 2003 is substantially different from Exchange 5.x OWA. Exchange 5.x OWA uses ASP pages to communicate with the Exchange Server computer that uses CDO 1.2 and MAPI. In this context, OWA is actually a part of Microsoft Internet Information Services (IIS). The reliance on MAPI-based access to the Exchange Server computer for accessing the mailbox store limits the effective number of people who can simultaneously use the service.
OWA in Exchange 2000 and in Exchange 2003 does not use MAPI to communicate with the mailbox store, and ASP pages are no longer used for client access. Clients continue to access the Web Access Component through HTTP; however, the IIS server hosting the component uses the new Web Store to provide access to the user's messaging functions.
IIS receives OWA client requests as a proxy for message traffic between a Web client and either an Exchange 2000 server or an Exchange 2003 server. IIS receives the client requests, looks at the namespace, and passes the client to the appropriate OWA scripts that are registered for that URL. If the server contains the Exchange 2000 database or the Exchange 2003 database, OWA uses a high-speed channel to access the mailbox store. If the server is a front-end server, OWA proxies the request to a back-end server by using HTTP.