Microsoft Exchange Server 2016: Mail Flow and the Transport Pipeline

 

This tutorial is about mail flow and the transport pipeline in Microsoft Exchange 2016.

 

  • In Exchange Server 2016, mail flow happens through the transport pipeline. The transport pipeline is a aggregation and collection of services, connections, components, and queues which work together to route all messages to the categorizer in the Transport service on an Exchange 2016 Mailbox server present inside the organization.

 

Objective of this tutorial

  • Understanding the transport pipeline
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  • How messages from external senders enter the transport pipeline
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  • How messages from internal senders enter the transport pipeline
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  • Understanding the Transport service on Mailbox servers
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  • Understanding the Transport service on Edge Transport servers

 

Understanding the transport pipeline

The transport pipeline is inclusive of the following services:

 

  • Front End Transport service present on Mailbox servers This service serves or acts as a stateless proxy for all inbound preferably and (optionally) outbound external SMTP traffic present for the Exchange 2016 organization. The Front End Transport service does not introspect message content, doesn’t connect to communicate with the Mailbox Transport service, and doesn’t queue any given messages locally.
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  • Transport service present on Mailbox servers This service is virtually similar to the Hub Transport server role on Exchange Server 2010.
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  • The Transport service controls and handles all SMTP mail flow for the enterprise, performs message segregation or categorization, and performs message content examination.
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  • Unlike Exchange 2010, the Transport service never effort to communicate directly with mailbox databases. And that respective task is now handled by the Mailbox Transport service.
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  • The Transport service direct and routes messages among the Mailbox Transport service, the Transport service, the Front End Transport service, and (based on your configuration) the Transport service on Edge Transport servers.

 

Mailbox Transport service present on Mailbox servers This service includes two separate services:

 

  • Mailbox Transport Submission service This said service connects to the local mailbox database by using an Exchange remote procedure call (RPC) to restore or retrieve messages.
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  • The service deposits/submits the messages over SMTP to the Transport service on the local Mailbox server or over other Mailbox servers. The Mailbox Transport Submission service has access eligibility to the same routing topology information as the Transport service.
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  • Mailbox Transport Delivery service This referred service receives SMTP messages from the Transport service present on the local Mailbox server or on other Mailbox servers and communicates or connects to the local mailbox database using RPC to deliver the messages.
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  • The Mailbox Transport service does not communicate with the Front End Transport service, and the Mailbox Transport service, or mailbox databases present on other Mailbox servers. This also doesn’t queue any messages locally.
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  • Transport service present on Edge Transport servers This service is very similar to the Transport service present on the Mailbox servers. In case you have an Edge Transport server installed in the perimeter network, all mail arriving from the Internet or going to the Internet flows via the Transport service Edge Transport server.

 

The shown below diagram shows the relationships between the components in the Exchange 2016 transport pipeline.

 

Kindly Note:

Though the diagrams in this topic depicts the components on a single Exchange 2016 server, communication also happens between those components on distinguish Exchange 2016 servers. The only communication which always occurs over the local Exchange 2016 server is among the Mailbox Transport service and the local mailbox database.

 

A Brief Overview of the transport pipeline in Exchange 2016

Overview of the transport pipeline in Exchange 2016
 

How messages via external senders enter the transport pipeline

 

  • The approach where messages from outside the Exchange organization enter the transport pipeline based on whether you have a subscribed Edge Transport server deployed in your perimeter network.

 

Inbound mail flow with Exchange 2016 (no Edge Transport servers)

 

  • The following diagram and list describe inbound mail flow with only Exchange 2016 Mailbox servers.

 
 inbound mail flow
 

  • A message from outside the enterprise enters the transport pipeline via the default Receive connector depicted with the name ‚ÄúDefault Frontend” in the Front End Transport service.
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  • The message is transmitted to the Transport service present over the local Mailbox server or on a variant Mailbox server. The Transport service receives or listens for messages on the default Receive connector named “Default “.
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  • The message is transmitted from the Transport service over to the Mailbox Transport Delivery service present on the local Mailbox server or on a distinguished Mailbox server.
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  • The Mailbox Transport Delivery service utilizes RPC to deliver the message over to the local mailbox database.

 

Inbound mail flow with Edge Transport servers

 

  • The shown below diagram and list illustrates inbound mail flow with an Edge Transport server setup and installed in the perimeter network

 

 inbound mail flow with an Edge Transport server setup

 

  • A message received from outside the Exchange enterprise enters the transport pipeline via the default Receive connector known with the name “Default internal Receive connector ” over the Transport service present on the Edge Transport server.
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  • In the Transport service present on the Edge Transport server, the default Send connector known with name “EdgeSync – Inbound to ” sends/transmits the message to a Mailbox server in the subscribed Active Directory site.
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  • In the Front End Transport service present on the Mailbox server, the automatic default Receive connector named “Default Frontend ” received/accepts the message.
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  • The message is sent/ transmitted from the Front End Transport service to the Transport service present on the local Mailbox server or on a distinguished Mailbox server. The Transport service listens/ receives for messages on the default Receive connector named “Default “.
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  • The message is sent/ transmitted from the Transport service to the Mailbox Transport Delivery service present on the local Mailbox server, or on a distinguish Mailbox server.
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  • The Mailbox Transport Delivery service utilizes RPC to deliver the message to the local mailbox database.

 

How messages received from internal senders enter the transport pipeline

 
SMTP messages received from inside the organization enter the transport pipeline via the Transport service present on a Mailbox server with one of the following ways:

 

  • Via a Receive connector.
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  • From the Pickup directory or the Replay directory.
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  • From the Mailbox Transport Submission service.
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  • Via an agent submission.
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  • The message routed depending on the routing destination or delivery group.

 

Outbound mail flow (no Edge Transport servers)

 

  • By default automatically, in a new Exchange 2016 organization, there’s no Send connector which is configured to send messages over to the Internet.

 

You must create the Send connector yourself. After that, Outbound mail flow happens as illustrated in the following shown diagram.

 

Outbound mail flow

 

  • The Mailbox Transport Submission service mainly uses RPC to retrieve or restore the outbound message from the local mailbox database.
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  • The Mailbox Transport Submission service mainly uses SMTP to send or transmit the message over to the Transport service present on the local Mailbox server or on a distinguish Mailbox server.
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  • In the Transport service, automatic default Receive connector named “Default ” receives or accepts the message.

 

The next step depends on the configuration of the Send connector:

Default

 

  • The Transport service mainly uses the Send connector you build or created to send the message to the Internet.

 

Outbound proxy

 

  • The Transport service mainly uses the Send connector you build to send the message over to the Front End Transport service present on the local Mailbox server or on a remote Mailbox server.
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  • In the Front End Transport service, the default Receive connector named “Outbound Proxy Frontend ” receives or accepts the message. The Front End Transport services transmit and send the message over to the Internet.

 

Outbound mail flow mainly with Edge Transport servers

 

  • In case you have an Edge Transport server setup and installed in the perimeter network, outbound mail never flows via the Front End Transport service.

 

Outbound mail flow with an Edge Transport server is illustrated in the following shown diagram below.

 

Outbound mail flow with an Edge Transport server

 

  • The Mailbox Transport Submission service mainly uses RPC to restore and retrieve the outbound message via the local mailbox database.
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  • The Mailbox Transport Submission service mainly uses SMTP to transmit the message over to the Transport service present on the local Mailbox server or on a distinguish Mailbox server.
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  • In the Transport service present on a Mailbox server in the subscribed Active Directory site, the automatic default Receive connector named “Default” accepts/receives the message.
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  • The message is sent or transmitted over to the Edge Transport server using the implicit and invisible intra-organization Send connector which automatically sends mail among Exchange servers in the same enterprises.
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  • In the Transport service over the Edge Transport server, the automatic default Receive connector named “Default internal Receive connector ” receives/accepts the message.
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  • Located in the Transport service present on the Edge Transport server, the default Send connector with name “EdgeSync – to Internet” transmits or sends the message to the Internet.

 

Learning and Understanding the Transport service on Mailbox servers

 

  • Every message which is sent or received in an Exchange 2016 organization should be categorized in the Transport service on a Mailbox server before it tend to be routed and delivered.
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  • After a message has been segregated and categorized, it is directed in a delivery queue for delivery to the destination mailbox database, the destination database availability group (DAG), Active Directory site or Active Directory forest, or to the destination domain present outside the organization.

 

The Transport service present on a Mailbox server includes the following components and processes:

 

SMTP Receive

 

  • Whenever messages are received by the Transport service, message content inspection is conducted, transport rules are applied, and anti-spam and anti-malware inspection is conducted if they are enabled.
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  • The SMTP session has a sequence and series of events that work together in a particular order to verify and validate the contents of a message before it’s accepted. When a message has passed absolutely through SMTP Receive and isn’t dejected by receive events, or via an anti-spam or anti-malware agent, it is directed over the Submission queue.

 

Submission

 

  • Submission is the process of directing messages into the Submission queue. The segregator or categorizer picks up single message at a time for categorization.

 

Submission happens in three ways:

 

  • From SMTP Receive via a Receive connector.
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  • Via the Pickup directory or the Replay directory. These directories present on Mailbox servers and Edge Transport servers. Accurately formatted message files which are copied into the Pickup directory or the Replay directory are routed directly into the Submission queue.
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  • Via a transport agent.

 

Categorizer

 

  • The categorizer chooses one message at a time from the Submission queue.

 

The categorizer performs the following steps:

 

  • Recipient resolution that includes top-level addressing,
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  • message bifurcation, and distribution group expansion.
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  • Routing resolution.
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  • Content conversion.

Moreover, mail flow rules which the organization defined are applied. After messages have been segregated, they’re routed into a delivery queue which is based on the destination of the message. Messages are lined up and queued by the destination mailbox database, Active Directory site, DAG, Active Directory forest, or external domain.

 

SMTP Send

 

  • How messages are directed and routed from the Transport service based on the location of the message recipients relative to the Mailbox server where categorization happened.

 

The message might be routed to one among the following locations:

 

  • Over to the Mailbox Transport Delivery service located on the same Mailbox server.
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  • Over to the Mailbox Transport Delivery service present on a different Mailbox server that’s part of the same DAG.
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  • Over to the Transport service over a Mailbox server in a distinguished DAG, Active Directory site, or Active Directory forest.

 

For delivery over to the Internet through:

 

  • A Send connector present on the same Mailbox server.
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  • The Transport service present on a different Mailbox server.
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  • The Front End Transport service present on the same Mailbox server or a distinguished Mailbox server (in case outbound proxy is configured).
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  • The Transport service present on an Edge Transport server present in the perimeter network.

 

Learning the Transport service on Edge Transport servers

 

  • The components of the Transport service present on Edge Transport servers are similar to the components of the Transport service on Mailbox servers. But, what actually happens during every stage of processing on Edge Transport servers is quite different.

 

The differences are illustrated in the following list.

 

SMTP Receive

 

  • Whenever an Edge Transport server is subscribed to an internal Active Directory site, then the default Receive connector with the name “Default ” is by default automatically configured to receive or accept mail from internal Mailbox servers and from the Internet.
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  • When the Edge Transport server receive an Internet messages, anti-spam agents filter connections and message contents and help to search the sender and the recipient while the message is being approved and accepted into the enterprise.
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  • The anti-spam agents are setup installed and enabled by default. Further, additional attachment filtering and connection filtering functional features are available; however built-in malware filtering is not. Mainly the transport rules are controlled by the Edge Rule agent.
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  • In comparison to the Transport Rule agent present on Mailbox servers, only a small subset of transport rule conditions is available on the Edge Transport servers. But, there are unique transport rule actions associated to SMTP connections which are available only over the Edge Transport servers over the system.

 

Submission

 

  • Over an Edge Transport server, messages generally enter the Submission queue via a Receive connector. But, the Pickup directory and the Replay directory are also available.

 

Categorizer

 

  • It present on an Edge Transport server, segregation or categorization is a short process in which the message is route directly towards a delivery queue for delivering it to internal or external recipients.

 

SMTP Send

 

  • When an Edge Transport server is subscribed over to an internal Active Directory site, two Send connectors are automatically by default created and configured.
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  • Single named “EdgeSync – to Internet” is responsible for transmitting outbound mail over to Internet recipients; the other named “EdgeSync – Inbound to ” is responsible for transmitting inbound mail from the Internet to internal recipients.
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  • Inbound mail is transmitted to the Front End Transport service on a present Mailbox server in the subscribed Active Directory site.

Kristin is a content strategist at Techarex Networks. Kristin follows the B2B technology space closely and loves to write on the latest changes in technology, futuretech and fixes for day to day how to issues. Besides writing Kristin also loves music, moves and skating.

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