Client Access Server (CAS) is a server role in Microsoft Exchange Server 2013 and before. It has been eliminated from Microsoft Exchange Server 2016 architecture and replaced with a Client Access Service running on the Mailbox server role. In Exchange Server deployments prior to the latest release, and in hybrid 2016 and previous version deployments, CAS provides the services that allow users to access mail and other services provided by Exchange Server. It is the server that clients communicate with, it runs Outlook Web App, and also the ActiveSync engine and protocol for syncing client mailboxes. The CAS also routes mail between Exchange Server and external mail systems on the Internet.