Servicing of the transport pipeline
Mail flow takes place in Exchange through the transport pipeline, which includes components, connections, services, and queues working collectively to route e-mails. With all the architectural changes introduced in Exchange 2013, three different services are part of the transport pipeline, and they are as follows:
Front End Transport service: Running on CASs, this service is a stateless proxy for the external SMTP traffic, which is both inbound and outbound. It communicates only with the Transport service on Mailbox servers, and does not inspect or queue any e-mails.
Transport service: Running on Mailbox servers, this service is similar to the previous Hub Transport server role. It handles the mail flow within the organization, and performs content-inspection and e-mail categorization. It is capable of queuing e-mails, but it does not communicate with mailbox databases; it simply routes e-mails between the Front End Transport service and the Mailbox Transport...
