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have removed the Client Access server (CAS) role and added the client access services to the Mailbox role. Even without the CAS role, the system maintains loose coupling in terms of functionality, versioning, user partitioning and geographical affinity.

The Mailbox server role now:

1. Houses the logic to route protocol requests to the correct destination endpoint.
2. Hosts all of the components and/or protocols that process, render and store the data.

No clients connect directly to the back-end endpoints on the Mailbox server; instead, clients connect client access services and are routed (via local or remote proxy) to the Mailbox server that hosts the active database that contains the user’s mailbox.

Mailbox servers can be added to a Database Availability Group (DAG), thereby forming a high availability unit that can be deployed in one or more datacenters. DAGs in Exchange Server 2016 do have a few specific enhancements:

1. DatabaseAvailabilityGroupIpAddresses is no longer required when creating a DAG. By default, the failover cluster will be created without an administrative access point, as this is the recommended best practice.
2. Replay Lag Manager is enabled by default.
3. Lagged database copy play down can be delayed based on disk latency, thereby ensuring active users are not impacted.
4. Database failovers times are reduced by 33% when compared to Exchange Server 2013.

Removal of the separate CAS role does not affect how communication occurs between servers. Communication between servers still occurs at the protocol layer, effectively ensuring that every server is an island. For a given mailbox’s connectivity, the protocol being used is always served by the protocol instance that is local to the active database copy.

Redmor

http://blogs.technet.com/b/exchange/archive/2015/05/05/exchange-server-2016-architecture.aspx

Taken from the site of Thanks Team Blog