Exchange Server 2013 Transport Services

Front End Transport service
This service runs on the Client Access server. This service acts as a stateless proxy component to all incoming and outgoing SMTP traffic that is external to the Exchange
The service accepts the SMTP connections from other SMTP servers on the Internet, receives messages, and initiates SMTP connections for message sending. However, this service cannot perform message queuing. This service is capable of filtering based on IP connections, domains, senders, or recipients. 
Internally, this service only communicates with the Hub Transport service that resides on the Mailbox Server role.
Transport service
This service is similar to the Hub Transport server role in Exchange Server 2007 and Exchange Server 2010. It runs on all of the Mailbox servers in an Exchange Server 2013 organization. 
This service handles all internal SMTP flow, and performs message categorization and content inspection. The most important difference between this service and the Hub Transport server role in previous Exchange versions is that the Hub Transport service, in Exchange Server 2013, never communicates directly with the mailbox databases. The Transport service routes messages between the Front End Transport service and the Mailbox Transport service. The Mailbox Transport service, in turn, communicates with the mailbox database.
Mailbox Transport service
Mailbox Transport service runs on a Mailbox Server role. This service has components as listed below:
  • Mailbox Transport Deliver: 

This service receives SMTP messages from the Hub Transport service and then establishes the Remote Procedure Call (RPC) connection to the mailbox database to deliver the message to the appropriate mailbox. 

  • Mail Transport Submission: 

This service works in the opposite direction of the Mailbox Transport Delivery service. While it also connects the RPC to the mailbox database, its purpose is to retrieve messages for sending rather than to deliver messages. It then submits the received messages to the Hub Transport service by using the SMTP protocol. Unlike the Hub Transport service, the Mailbox Transport service cannot perform local message queuing.

Message Flow:
  1. Messages that are coming from the Internet enter the Exchange transport pipeline through a Receive connector on the Front End Transport service on a Client Access server. 
  2. Later messages are routed to the Hub Transport service on a Mailbox server. Messages inside the organization come directly to the Hub Transport service on a Mailbox server, through the Receive connector, the Mailbox Transport service, or the agent submission.
If you have an Edge Transport server deployed in your perimeter network, Internet mail flow occurs directly between the Hub Transport service on the Mailbox server and the Edge Transport server, without passing through Front End Transport on Client Access server.

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