The client access server (CAS) is a server role that handles all client connections to Exchange Server 2010 and Exchange 2013. The CAS supports all client connections to Exchange Server from Microsoft Outlook and Outlook Web App, as well as ActiveSync applications. The CAS also provides access to free/busy data in Exchange calendars.
The CAS is one of five server roles in Exchange Server 2007 and Exchange 2010, and one of two server roles in Exchange Server 2013. It must be installed in every Exchange Server organization and on every Active Directory (AD) site that has the Exchange mailbox server role installed. In Exchange 2007, Outlook client connections were handled by the mailbox server.