Note: So what is the Client Access server in Exchange 2013? The Client Access server role is comprised of three components, client protocols, SMTP, and a UM Call Router. The
CAS role is a thin, protocol session stateless server that is organized into a load balanced configuration. Unlike previous versions, session affinity is not required at the load balancer
(but you still want a load balancer to handle connection management policies and health checking). This is because logic now exists in CAS to authenticate the request, and then route the request to the Mailbox server that hosts the active copy of the mailbox database.
The Mailbox server role now hosts all the components and/or protocols that process, render and store the data. No clients will ever connect directly to the Mailbox server role; all client connections are handled by the Client Access server role.
Reference: Exchange 2013 Server Role Architecture