The Microsoft Exchange Server version 5.5 Enterprise Edition includes many enhancements that provide support for Microsoft Cluster Server version 1.0. Clustering your Microsoft Exchange Server computers ensures that messaging services remain uninterrupted even if one of the clustered servers fails. For example, if the processor in a clustered Microsoft Exchange Server computer fails, another server in the cluster is available in its place. Users on the failed server do not see a change in their e-mail service. They can continue to send and receive mail, and browse public folders as usual.
Clustering is also convenient if you need to upgrade a server or add new hardware without interrupting service to users. Services can fail over to another server in the cluster so the other server takes over the functions of the server you are upgrading.
The following Microsoft Exchange Server components are not supported in a cluster environment.
Note Clustering is supported only in the Microsoft Exchange Server Enterprise Edition.