Understanding transport pipeline in Exchange server 2016

Mail flow occurs through the transport pipeline in Exchange 2016. The transport pipeline is a collection of services, connections, components, and queues that work together to route all messages to the categorizer in the Transport service.

Transport pipeline consists of the following services:

           Front End Transport service                                                            -FETS
           Transport service                                                                                    -TS        
           Mailbox Transport service                                                                -MTS
Mailbox Transport Submission service               -MTSS
Mailbox Transport Delivery service                       -MTDS

           Transport service on Edge Transport servers 

Front End Transport service

FETS acts as a stateless proxy for all inbound and outbound SMTP traffic, it will not inspect message content, and will not communicate with the Mailbox Transport service, and doesn't queue any messages locally.

Transport service on Mailbox servers  

TS routes messages among the MTS and FETS, this service is identical to the Hub Transport server role in Exchange Server 2010. It handles all SMTP mailflow for the organization, performs message categorization and content inspection, TS never communicates directly with mailbox databases. That task is now handled by the MTS.

Mailbox Transport service

      This service consists of two separate services:
                   Mailbox Transport Submission service: Connects to the local mailbox DB using  RPC to retrieve messages and submits the messages over SMTP to the TS. The MTSS has access to the same routing topology information as the Transport service.

                  Mailbox Transport Delivery service: This service receives SMTP messages from the TS and connects to the local mailbox database using RPC to deliver the messages. It also doesn't queue any messages locally.

Transport service on Edge Transport servers  

 This service is very similar to the TS on Mailbox servers. If you have an Edge Transport server installed in the perimeter network, all mail coming from the Internet or going to the Internet flows through the Transport service on Edge Transport server.

Source : https://technet.microsoft.com/en-us/library/aa996349(v=exchg.160).aspx