In this section, you will learn how to relay emails through Exchange 2013/2016 servers to internal and external recipients. In Exchange 2013, the Client Access Server role runs a service called Frontend Transport service, which is responsible for email filtering using antispam agents and routing of emails between your Exchange organization and the outside world. The architecture of Exchange 2016 has been further simplified, and it has only the Mailbox server role with required transport services that were earlier handled by the Client Access and Mailbox Roles in Exchange 2013.
There are two additional Transport services hosted by the Mailbox Server roles in Exchange 2013. In Exchange 2016, you will find all these services on the Mailbox Server role: