The primary role of the Mailbox server is to host mailbox databases where the recipient mailboxes are hosted. But in Exchange Server 2013 there’s more done besides hosting those mailboxes. The Hub Transport service is also running on the Mailbox server, and this service is responsible for all SMTP message routing in the Exchange organization. The Unified Messaging service is also part of the Mailbox server role, and there's a Client Access service running on the Mailbox server as well. As explained in the previous chapter, the Client Access server is a stateless protocol proxy, proxying user requests to the Mailbox server.