Due the architectural change, Mail flow in Exchange 2013 occurs VIA Transport Pipeline, it is collection of Transport services, connections, components and queues that work together to route the messages to the categorizer in the transport service on a MAILBOX server inside the organization.

Messages from outside organization enter the transport pipeline through a receive connector in the Front End Transport service on a client access server and then routed to the Transport Service on a Mailbox Server and the Mailbox Transport DELIVERY Service deliver the email to local mailbox database

Message from inside organization enter the Transport Service on a Mailbox Server in following ways, receive connector, Pick Up or Replay Directory, Mailbox Transport Service or AGENT Submission. Those emails can be relayed to Front End transport service on Client Access Server using the Transport Service on Mailbox Server and sent outside

Due the architectural change, Mail flow in Exchange 2013 occurs via Transport Pipeline, it is collection of Transport services, connections, components and queues that work together to route the messages to the categorizer in the transport service on a mailbox server inside the organization.

Messages from outside organization enter the transport pipeline through a receive connector in the Front End Transport service on a client access server and then routed to the Transport Service on a Mailbox Server and the Mailbox Transport Delivery Service deliver the email to local mailbox database

Message from inside organization enter the Transport Service on a Mailbox Server in following ways, receive connector, Pick Up or Replay Directory, Mailbox Transport Service or Agent Submission. Those emails can be relayed to Front End transport service on Client Access Server using the Transport Service on Mailbox Server and sent outside