The mailbox server role in Exchange 2013 hosts mailbox databases and other items, such as Client Access protocols, Transport service and Unified Messaging. All processing of a specific mailbox takes place on the Mailbox server hosting an active copy of the mailbox. Client connectivity is handled by the Client Access role, which can be co-located on the same server. But it only proxies the connections to Mailbox server components for further processing and rendering of the mailbox data being requested.
Important: Have you read page xvii?
It contains valuable information regarding the skills you need to pass the exam.
The Mailbox role is also expanded in Exchange 2013 to include Public ...