Microsoft Exchange Server 2013, mail flow occurs through the transport pipeline. 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 on a Mailbox server inside the organization.

The transport pipeline consists of the following services:

• Front End Transport service   This service runs on all Client Access servers and acts as a stateless proxy for all inbound and outbound external SMTP traffic for the Exchange 2013 organization. The Front End Transport service doesn’t inspect message content, but it can filter messages based on connections, domains, senders, and recipients. The Front End Transport service only communicates with the Transport service on a Mailbox server, and doesn’t queue any messages locally.
• Transport service   This service runs on all Mailbox servers and is virtually identical to the Hub Transport server role in previous versions of Exchange. The Transport service handles all SMTP mail flow for the organization, performs message categorization, and performs message content inspection. Unlike previous versions of Exchange, the Transport service never communicates directly with mailbox databases. That task is now handled by the Mailbox Transport service. The Transport service routes messages between the Mailbox Transport service, the Transport service, and the Front End Transport service.
• Mailbox Transport service   This service runs on all Mailbox servers and consists of two separate services: the Mailbox Transport Submission service and Mailbox Transport Delivery service. The Mailbox Transport Delivery service receives SMTP messages from the Transport service on the local Mailbox server or on other Mailbox servers, and connects to the local mailbox database using an Exchange remote procedure call (RPC) to deliver the message. The Mailbox Transport Submission service connects to the local mailbox database using RPC to retrieve messages, and submits the messages over SMTP to the Transport service on the local Mailbox server, or on other Mailbox servers. The Mailbox Transport Submission service has access to the same routing topology information as the Transport service. Like the Front End Transport service, the Mailbox Transport service also doesn’t queue any messages locally.

Messages from outside the organization enter the transport pipeline through a Receive connector in the Front End Transport service on a Client Access server and are then routed to the Transport service on a Mailbox server.

Messages inside the organization enter the Transport service on a Mailbox server in one of the following ways:

• Through a Receive connector.
• From the Pickup directory or the Replay directory.
• From the Mailbox Transport service.
• Through agent submission.

The following figure shows the relationships among the components in the Exchange 2013 transport pipeline.

Every message that’s sent or received in an Exchange 2013 organization must be categorized in the Transport service on a Mailbox server before it can be routed and delivered. After a message has been categorized, it’s put 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 outside the organization.

The Transport service on a Mailbox server consists of the following components and processes:

• 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. The SMTP session has a series of events that work together in a specific order to validate the contents of a message before it’s accepted. After a message has passed completely through SMTP Receive and isn’t rejected by receive events, or by an anti-spam and anti-malware agent, it’s put in the Submission queue.
• Submission   Submission is the process of putting messages into the Submission queue. The categorizer picks up one message at a time for categorization. Submission happens in three ways:
  • Through an SMTP Receive connector.
  • Through the Pickup directory or the Replay directory. These directories exist on the Mailbox server. Correctly formatted message files that are copied into the Pickup directory or the Replay directory are put directly into the Submission queue.
  • Through a transport agent.
• Categorizer   The categorizer picks up one message at a time from the Submission queue. The categorizer completes the following steps:
  • Recipient resolution, which includes top-level addressing, expansion, and bifurcation.
  • Routing resolution.
  • Content conversion.

  Additionally, mail flow rules that are defined by the organization are applied. After messages have been categorized, they’re put into a delivery queue that’s based on the destination of the message. Messages are queued by the destination mailbox database, DAG, Active Directory site, Active Directory forest or external domain.

• SMTP Send   How messages are routed from the Transport service depends on the location of the message recipients relative to the Mailbox server where categorization occurred. The message could be routed to the Mailbox Transport service on the same Mailbox server, the Mailbox Transport service on a different Mailbox server that’s part of the same DAG, the Transport service on a Mailbox server in a different DAG, Active Directory site, or Active Directory forest, or to the Front End Transport service on a Client Access server for delivery to the Internet.

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