With the new Exchange 2013 architecture moving away from the use of a Hub Transport server and only going with a Client Access server for front end transport and a Mailbox server some are confused regarding how mail flows.
The Microsoft Exchange Team says the mail flow process occurs through the “transport pipeline” which is made up of three services. These services aid in transport on our Client Access and Mailbox servers (which may exist on the same server). The Client Access has the Front End Transport service while the Mailbox server has the Hub Transport service and Mailbox Transport service (which is made up of two services).
The Front End Transport service on the Client Access server handles the flow of mail from the Mailbox server (specifically the Hub Transport service on the Mailbox server side) to the outside world. The Hub Transport service (or just Transport service) handles routing from the Front End Transport service to the Mailbox Transport service as well as between other servers within the organization internally. The Mailbox Transport service handles mail transport between the Hub Transport service and the mailbox database.
Going back to the Front End Transport service it’s basically a stateless proxy for inbound and outbound traffic with no traffic being queued as a result of that service, however, as mentioned, it can be used to filter traffic. That filtering can be based upon connections, domains, senders and recipients. It does inspect message content however. Inspection of the content itself can be done by the Transport service as it handle SMTP mail flow from the Front End Transport service to the Mailbox Transport service and into the database.
Note: One important point regarding the Front End Transport service is that mail inbound and outbound to the Internet through an Edge server will bypass this service. The Edge will communicate directly with the Transport service on the Mailbox server.
The two services that make up the Mailbox Transport service include the Mailbox Transport Submission service and the Mailbox Transport Delivery service. The Delivery side accepts messages from the Transport service and delivers them using RPC to the mailbox database. And the Submission service receives, through RPC, messages from the local mailbox database and passes it to the Transport service.
If a graphical representation will help make all of this come to life here is an artists rendition of the process.