CLUSTERS
In the following figure, there are six servers -- two servers used primarily for mail and four servers used for applications. Some databases, such as DB1, are more important than others, so they have more replicas. In this scenario, you can replicate all of the mail databases from one mail server to the other mail server, if the servers are powerful enough to handle the load if failover occurs. If the two mail servers are not powerful enough, you can distribute some mail replicas to one or more of the other servers in the cluster.