I want to install Exchange Server 2003 back-end servers in my existing Exchange 2000 Server environment, which contains front-end and back-end servers, but Exchange Setup won't let me. What steps must I perform to make this configuration work?

The front-end and back-end topology is useful in a lot of circumstances, but it poses some challenges for migrating to Exchange 2003. Whereas Exchange 2003 and Exchange 2000 can coexist in one Exchange organization, you must follow certain rules when mixing the two versions within the same administrative group. You've already discovered one rule: You can't install any Exchange 2003 back-end servers into an administrative group that contains Exchange 2000 front-end servers. In addition, if you install an Exchange 2003 server and use the server's Properties dialog box to configure it to be a front-end server, you can't install an Exchange 2003 front-end server in the same group—you must first install the server as a back-end server. Exchange Setup blocks this installation to avoid problems with front-end to back-end traffic between the versions. If you want to add Exchange 2003 back-end servers into an administrative group, you must first upgrade or remove all Exchange 2000 front-end servers in that administrative group. After you complete this step, you'll be able to install Exchange 2003 back-end servers. Be forewarned that you might still encounter problems in some configurations. For example, let's say that you have two administrative groups called AG1 and AG2. Suppose you upgrade all AG1 front-end servers to Exchange 2003. If a user whose mailbox is on an Exchange 2000 server in AG2 connects through an AG1 front-end server, that user might experience Outlook Web Access (OWA) related errors.