As you might have seen in the Exchange 2013 Mail Flow article, the Transport Pipeline is now made of three main services:
- Front End Transport service, which runs on all Client Access servers (CAS) and acts as a stateless proxy for all inbound and outbound external SMTP traffic;
- Transport service which runs on all Mailbox servers and is almost identical to the Hub Transport server role in previous versions of Exchange;
- Mailbox Transport service which also runs on all Mailbox servers and is made of two separate services, the Mailbox Transport Delivery service and the Mailbox Transport Submission service.
Because of this change in architecture, the message headers in Exchange 2013 have been updated to include information regarding which service(s) dealt with the message, making it easier to troubleshoot any possible issues.
For example, if we look at an e-mail received from the Internet, into a CAS server and then delivered to a user’s mailbox, we can see that both the Front End Transport service and the Mailbox Transport service dealt with the message:
On the other hand, if we look at an internal e-mail, we do not see the Front End Transport service as the e-mail does not go through this service if it is from an internal user to another internal user: