Microsoft has mixed server roles in Exchange server 2013 as details are given below
Client Access Server: The CAS role allows for client connection such as OWA, Exchange Active Sync, PO3 , IMAP and Mapi (Outlook 2010, 2003, 2013). the best thing I noticed is the CAS servers are totally stateless so it can be load-balanced for backup purposes using any number of load-balancing technologies. The CAS roles can co-exists with other roles for smaller organizations with a single server.
Mailbox Server :Mailbox server is storage for users email boxes. All connections to the mailbox server are proxy through CAS server. basically it contains mailbox and public folder Data bases. mailbox server also runs two transport services
Hub Transport Service- similar to the old versions of 2007 and 2010 exchange. the function of this service is to route emails with in the organization, and connectivity between the Front end transport service and the mailbox transport service.
Mailbox Transport Service- this service routes email messages between the hub transport services and the Mailbox database.
Edge Transport Server-this is unchanged same as exchange 2010 role. it acts as a gateway for SMTP traffic for messages and recipients filtering
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