Microsoft Exchange Server 2010 is a collaborative software from Microsoft Corporation that runs on the Windows Server operating system. Its can function as a mail server, a contact manager, and a calendaring software. Exchange Server 2010 has new features that enhance its efficiency. CCR, SCC, LCR, and site functionality SCR has been replaced by DAG (Data Availability Groups) to provide a higher level of availability for database (as opposed to server level availability). As a result database copies may be removed or added without requiring major reconfiguration of the server. Another feature is the use of Client Access Server (CAS) arrays. Multiple Client Access Servers are contained in an array, with a single identification in an Active Directory. Thus, faster client connection is effected by the single name endpoint. In previous editions of the Microsoft Exchange Server, a clustered mailbox server cannot be combined with another role. In Exchange Server 2010 users can combine the Mailbox Server Role with the Client Access Server and Hub Transport roles.