Exchange 2016 Mail Flow

Now that we have installed Exchange 2016, let’s discuss the details of how we are receiving messages. Before we get too carried away, we need to understand the transport pipeline. The transport pipeline consists of several services that are used to route messages around the inside of the organization. The services are:

  • Front End Transport Service on Mailbox servers. This service is the front line of the mailbox server. The service is a stateless connection for inbound and (if configured) outbound SMTP connection. This service does not inspect or queue any messages.
  • Transport Service on Mailbox servers. This service is essentially the same as the hub transport service in Exchange 2010. This service receives messages from the front end transport service. It performs content inspection, message queueing, and categorization. The only real difference between the transport service and the hub transport service is that the transport service does not communicate directly with the mailbox databases. That task is now handled by the mailbox transport service.
  • Mailbox Transport Service on Mailbox servers. This service is made up of two separate services.
    • Mailbox Transport Submission Service. This service uses the RPC protocol to connect to the mailbox and is used to deliver messages from the mailbox to the transport service. (Sending Emails)
    • Mailbox Transport Delivery Service. This services uses the RPC protocol to connect to the transport service and is used to deliver messages to the mailbox.
It should also be noted that there is also the Transport Service on Edge Transport servers. But for our purposes we are only concerned with the mailbox server.
Now that we have a familiarity with the services involved, let’s look at the actual receive connectors that are installed by default. To find these connectors, log in to the Exchange Admin Center (EAC), click mail flow, and select receive connectors:
screen-shot-2016-10-27-at-8-25-08-pm
Alternatively, you can use the Exchange Management Shell and run the following command:
[PS] C:Windows\system32> Get-ReceiveConnector
Now that we have found the receive connectors, let’s look at what each of them do:
  • Client Frontend (Frontend Transport). This connector is the frontend connector that accepts connections using TLS on port 587 for POP and IMAP clients. It then sends to connection to the client proxy hub transport connector.
  • Client Proxy (Hub Transport). This connector accepts connections from a frontend service on port 465 for POP and IMAP connections. It then delivers the messages to the mailbox transport service.
  • Default (Hub Transport). This connector accepts connections over port 2525 from a frontend service to route the message to the mailbox transport service or from the mailbox transport service to route the message outbound. If the send connector is configured to do so, it can also send the outbound message to the front end transport service outbound proxy frontend.
  • Default Frontend (Frontend Transport). This connector is the frontend connector that accepts messages on port 25 from the internet. It then delivers the messages to the default hub transport service on port 2525.
  • Outbound Proxy Frontend (Frontend Transport). If you have configured your send connector to proxy through the client access server then the message will be sent from the default hub transport connector to this connector on port 717 and then sent outbound.
This should give you a pretty good base understanding of the default Exchange 2016 receive connectors.