Understanding the Server Roles in Exchange Server 2013

           Exchange Server 2013, the Mailbox Server role also includes Client Access protocols, Hub Transport service, mailbox
databases, and Unified Messaging components.

Client Access Server
           Is a proxy server that handles all client connections, by admitting all client requests and routing them to the correct active Mailbox database. It provides authentication, redirection, and proxy services, and offers support for the following client access protocols: HTTP, POP and IMAP, and SMTP.

Mailbox Server
           Provides much more functionality than in previous Exchange Server versions. This includes integration of the Hub Transport service (previously known as the Hub Transport server role) and Unified Messaging service (previously known as the Unified Messaging server role). This is the key role for storing mailbox and public folders data, as well as for Unified Messaging functionality and message queuing.

Client and Server Communication in Exchange Server 2013
           The most important connectivity change is that remote procedure call (RPC) is no longer supported as a direct client access protocol. All client connections are established by using RPC over HTTPS, means that all clients are connecting by using the Outlook Anywhere service.
This eliminates the need to have the RPC service running on the Client Access server. In addition, you will have one fewer FQDN to manage, because all clients will be using a new connection point made up of the user’s mailbox GUID + @ + UPN suffix. As a result of these changes, only Outlook 2007 and newer clients support connection to Exchange Server 2013.