6.4 Mixed Exchange 5.5 and Exchange 2000 environments

Many of the new features included with Exchange 2000 are only available if you are operating in an environment that includes only Exchange 2000 servers and are running in native mode. However, if you are migrating from Exchange 5.5 to Exchange 2000, it is unlikely that you will simultaneously upgrade all of your existing Exchange servers. Instead, you will be left with an Exchange organization that includes a mixture of Exchange 5.5 and Exchange 2000 servers.

Even if you only have a small Exchange 5.5 environment, the duration of your migration to a native mode Exchange 2000 environment may take longer than you expect since some of the connectors that allow you to exchange e-mail with other systems may not be available for Exchange2000. You will need to keep these connectors running on Exchange 5.5 servers.

As long as you have Exchange 5.5 servers in your organization, you must run your Exchange 2000 servers in mixed mode to provide full compatibility and coexistence with the older version of Exchange. In mixed mode, you can take advantage of some new Exchange 2000 features, but some features can only be used in native mode.

In mixed mode, your use of Exchange 2000 administrative groups and routing groups is limited so that the Exchange 2000 servers can work...