Front End Transport Service in Client Access Server acts as a stateless proxy for all inbound and outbound external SMTP traffic for Exchange 2013 organization. It won’t inspect MESSAGE content, only communicates with the Transport Service on a MAILBOX Server, and doesn’t queue any messages locally.

Transport Service in Mailbox server is identical to Hub Transport server, it handles all SMTP mail flow for the organization, perform message categorization, and performs content inspection. It won’t communicate DIRECTLY with Mailbox Database that task will be handled by Mailbox Transport Service. So the Transport Service routes messages between the mailbox transport service, the transport service and the front end transport service.

Mailbox Transport Service RUNNING on Mailbox server consists of two separate services: the Mailbox Transport Submission Service and Mailbox Transport Delivery Service.

Mailbox Transport Delivery Service receives emails from Transport Service on the local or different mailbox server and connects to the local mailbox databases USING Exchange RPC to deliver the message

Mailbox Transport Submission Service connects to local mailbox database using RPC to retrieve messages and submits the messages over SMTP to the Transport Service on the local Mailbox server or other Mailbox Servers.

Front End Transport Service in Client Access Server acts as a stateless proxy for all inbound and outbound external SMTP traffic for Exchange 2013 organization. It won’t inspect message content, only communicates with the Transport Service on a Mailbox Server, and doesn’t queue any messages locally.

Transport Service in Mailbox server is identical to Hub Transport server, it handles all SMTP mail flow for the organization, perform message categorization, and performs content inspection. It won’t communicate directly with Mailbox Database that task will be handled by Mailbox Transport Service. So the Transport Service routes messages between the mailbox transport service, the transport service and the front end transport service.

Mailbox Transport Service running on Mailbox server consists of two separate services: the Mailbox Transport Submission Service and Mailbox Transport Delivery Service.

Mailbox Transport Delivery Service receives emails from Transport Service on the local or different mailbox server and connects to the local mailbox databases using Exchange RPC to deliver the message

Mailbox Transport Submission Service connects to local mailbox database using RPC to retrieve messages and submits the messages over SMTP to the Transport Service on the local Mailbox server or other Mailbox Servers.