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Exchange 2000 and Exchange 2003 are backward compatible with Exchange 5.5 through the use of the Site Replication Service (SRS). When Microsoft Exchange is running in mixed mode, the Exchange 2000/2003 servers will peacefully co-exist with Exchange 5.5 servers in a common Exchange organization. There are a few issues you need to be aware of though.
Management consoles
Rule number one for mixed mode Exchange Server organizations is that you can't mix management tools. You can't use Exchange Administrator to manage an Exchange 2000/2003 server, and you can't use Exchange System Manager (ESM) to manage an Exchange 5.5 server.
Exchange System Manager will display Exchange 5.5 servers as a part of the Exchange organization. But they will be displayed as read only to prevent administrators from modifying them through ESM. Likewise, the Exchange Administrator console will display both Exchange 5.5 and Exchange 2000/2003 servers.
Unfortunately, the Exchange Administrator console doesn't contain any safeguards to prevent you from performing management actions against Exchange 2000/2003 servers. You just have to remember not to use it to manage your Exchange 2000/2003 servers.
System folders and public folders
Moving system folders and public folders works differently in Exchange mixed mode than it does in native mode. Because of problems with mail delivery to moved folders, Microsoft has disabled the ability to move system folders and public folders between routing groups in mixed mode. You can only move system folders and public folders across routing groups in native mode.
Administrative groups
In native mode, you can move mailboxes from one administrative group another. To the best of my knowledge, this feature was previously unavailable in mixed mode, until Microsoft enabled it in Exchange Server 2003 Service Pack 1. But in mixed mode, the administrative groups are still limited by the existing routing group structure (remember that Exchange 5.5 uses sites instead of routing groups); they cannot span multiple routing groups.
Query-based distribution groups
The major Exchange 2003 feature that you really miss out on if you run Microsoft Exchange in mixed mode is query-based distribution groups, which allow you to create dynamic distribution groups based on the results of an LDAP query against Active Directory.
For example, you could create a distribution group that contains all employees who work in the finance department. Because of this function's Active Directory dependency, it does not work in mixed mode.
About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server, Exchange Server and IIS. Brien has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at http://www.brienposey.com.
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Related information from SearchExchange.com:
Learning Guide: Exchange Server migration
Tip: From Exchange 5.5 to 2003 in a single bound
Reference Center: Exchange migration tips and resources
