Understanding the Transport service on Mailbox servers

In Exchange 2016 every message that is send or received, must be categorized in the Transport service before it can be routed and delivered. After the message has been categorized, it's put in a delivery queue for delivery to the destination.
(Mailbox database, DAG, AD SITE or AD forest, External Delivery outside the organization)
The Transport service consists of the following components and processes:
·        SMTP Receive
·        Submission
·        Categorization
·        SMTP Send
Let’s look at them one by one:
SMTP Receive: 
When messages are received by the Transport service, message content inspection is performed, transport rules are applied, and anti-spam and anti-malware inspection is performed if they are enabled.
If the message isn't rejected by receive events, or by an anti-spam or anti-malware agent, it's put in the Submission queue.
Submission: 
In the Submission process messages is placed into Submission queue, from here categorizer picks up the message one at a time.
Submission happens in three ways:
·        From SMTP Receive through a Receive connector.
·        Through a Pickup or Replay directory. They exist on Mailbox servers and Edge Transport servers.  
Correctly formatted message files that are copied into the Pickup or Replay directory are put directly into the Submission queue.
·        Through a transport agent.
Categorizer: 
Categorizer grab message one at a time from the Submission queue.
The categorizer do the following:
·        Recipient resolution, which includes top-level addressing, distribution group expansion, and message bifurcation.
·        Routing resolution.
·        Content conversion.
Additionally organization defined mail flow rules are applied.
After categorization message is put into the delivery queue based on the destination.
(Destination mailbox database, DAG, AD site, AD forest, or external domain)
SMTP Send:
Messages routing from the Transport service is depends on the location of the message recipients.
Message could be routed to one of the following locations:
·         To the Mailbox Transport Delivery service on the same Mailbox server.
·         To the Mailbox Transport Delivery service on a different Mailbox server that is part of the same DAG.
·         To the Transport service on a Mailbox server in a different DAG, AD site, or AD forest.
·         To the Internet through:
·         A Send connector on the same Mailbox server.
·         The Transport service on a different Mailbox server.
·         The Front End Transport service on the same Mailbox server or a different Mailbox server (if outbound proxy is configured).
·         The Transport service on an Edge Transport server in the perimeter network.
Source: https://technet.microsoft.com/en-us/library/aa996349(v=exchg.160).aspx

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