Overview of Mailbox Server Role in Exchange Server 2013

          In Exchange Server 2013, the Mailbox server hosts databases and provides email storage. It also hosts Client Access protocols, Transport service components, mailbox databases, and Unified Messaging components.
          However clients never communicate directly with the Mailbox server, this server interacts actively with the Active Directory Domain Services components and Client Access
server. It uses the Lightweight Directory Access Protocol to locate and access information about
recipients, servers, and organization configuration information that is stored in AD DS.
          The Mailbox server also participates in high-availability configurations through Database Availability Groups (DAGs). This concept provides high availability at a database level by implementing multiple copies on the same database over different mailbox servers. A DAG is a group of up to 16 Mailbox servers that hosts a set of databases and provides automatic database-level recovery from failures that affect individual servers or databases.

          The Mailbox server role in Exchange Server 2013 includes the following new features:

• In an evolution of the Exchange Server 2010 DAG, the transaction log code has been refactored for fast failover, with deep checkpoints on passive database copies.
• Servers can be in different locations to support enhanced site resiliency.
• Exchange Server 2013 now hosts some Client Access components, including the transport components and the Unified Messaging components.
• The Exchange store has been rewritten in managed code to improve performance in additional I/O reduction and reliability.
• Each Exchange Server 2013 database now runs under its own process.