In "How to Remove the Last Exchange 5.5 Server," March 2002, InstantDoc ID 23769, Daragh Morrissey described several pitfalls you need to avoid when deleting an Exchange Server 5.5 server from your organization. If you need to ensure that no disruption of Outlook Web Access (OWA) occurs in your environment, you'll need to complete one more task before you retire that old Exchange 5.5 server.
My company's Exchange 5.5 sites have a dedicated OWA 5.5 server that points to an Exchange 5.5 mailbox server. We introduced an Exchange 2000 Server system in this site and used the Move Mailbox method to migrate the users from the Exchange 5.5 server. We wanted to keep the OWA 5.5 server running until we found a different front-end/back-end strategy to use OWA 2000 features.
An OWA 5.5 server can communicate with an Exchange 2000 server to access mailboxes, so we navigated to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeWEB\Parameters registry subkey and modified the Server value from the name of the old Exchange 5.5 server to the name of the Exchange 2000 server. (We performed this change on the Exchange 5.5 OWA server.) The Microsoft article "XWEB: OWA Does Not Start When Server Is Removed from Site" (http://support.microsoft.com/default.aspx?scid=kb;en-us;q244850) discusses this change. After we made this change, we safely retired the Exchange 5.5 mailbox server without any disruption to OWA access.
If you decide to implement this solution, you need to be aware of a couple of caveats. First, you can't perform a fresh installation of a dedicated OWA 5.5 server and point it to an Exchange 2000 server (when prompted during setup). OWA setup will fail because OWA expects to access an Exchange 5.5 server. As a workaround, you can later upgrade an Exchange 5.5 server to an Exchange 2000 server, and OWA 5.5 will work.
Second, we noticed a behavior problem that occurs when an OWA 5.5 server points to an Exchange 2000 server. Suppose you have two Exchange 5.5 sites, one in London and one in Los Angeles, and each site has a dedicated OWA server. If a user whose mailbox resides in London travels to Los Angeles and uses the Los Angeles OWA server to access his or her mailbox, the Los Angeles OWA server will connect him or her smoothly by redirecting to the appropriate London Exchange 5.5 server. However, if you've changed the Los Angeles OWA 5.5 server to point to an Exchange 2000 server as part of an upgrade procedure, the London user won't be able to use the Los Angeles OWA 5.5 server to reach his or her mailbox. The only users who can access the Los Angeles OWA 5.5 server are those whose mailboxes reside on the Exchange 2000 server to which the OWA 5.5 server points. The London user must use the London OWA 5.5 server from Los Angeles. This workaround isn't a big deal, but it requires user training and awareness.