There are two different versions of Exchange Server 2010 that are available for purchase: Standard and Enterprise.

Exchange Server 2010 Standard Edition

Exchange Server 2010 Enterprise Edition

  • Hosts up to 100 databases

Simply put use Standard Edition for all Client Access, Hub Transport, Edge Transport, and Unified Messaging servers.

Mailbox servers that are members of a Database Availability Group need to count each passive copy of a database counts as a database on the server.  For example, if a Mailbox server has 3 active mailbox databases and another 3 passive copies, it will need to run Enterprise because the total is 6 over the 5 limit of Standard.

Exchange Server Enterprise Edition is not required for DAG members that host a total of 5 or fewer active and passive databases.  However, DAG members must run Windows Server editions with Failover Cluster feature (i.e. Windows Server 2008 R2 Enterprise or Windows Server 2012 Standard).  Note: Windows Server 2012 has been simplified into 2 editions, Standard and Datacenter.  Both editions include all features and roles of one another, the only difference is virtualization licensing rights.  Standard includes licenses for 2 VMs and Datacenter includes unlimited licenses for VMs.

The edition of Exchange Server 2010 is determined solely by the product key that is entered after the server is installed.  A server can be upgraded to Enterprise Edition conveniently by entering the new product key.