How the Mailbox Server Role Interacts with Clients and the Client Access Server

            To communication with AD DS, the Mailbox server role communicates intensively
with the Client Access server. It always takes the same paths, even when the Client Access server role is installed on the same server as the Mailbox server role.
            Clients never communicate directly
with the Mailbox server, so Client Access server
accepts client requests and sends them to the
Mailbox server. The Front End Transport service,
which runs on the Client Access server, accepts
and sends messages from the Internet, and then
forwards them to the Hub Transport service running on the Mailbox server.
            Client Access server also returns the data (content of the client mailbox) from the Mailbox server to the clients. In addition, the Client Access server uses NETBIOS file sharing to access the offline address book (OAB) data from the Mailbox server role. This data is then served to the clients through the OAB virtual directory on the Client Access server. The Client Access server also sends messages, free/busy data, and client profile settings between the client server and the Mailbox server.
            In Exchange Server 2013, both internal and external client communication is proxied through the Client Access server. The Client Access server uses LDAP or the Name Service Provider Interface (NSPI) to contact the Active Directory server and retrieve the user’s Active Directory information.