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Understanding Transport Pipeline

Applies to: Exchange Server 2010

Topic Last Modified: 2010-03-16

In Microsoft Exchange Server 2010, the transport pipeline is a collection of Exchange 2010 server roles, connections, components, and queues that work together to route all messages to the categorizer on a Hub Transport server inside the organization. Messages from outside the organization enter the transport pipeline through a Receive connector on an Edge Transport server and are then routed to a Hub Transport server inside the organization. Messages inside the organization enter the transport pipeline on a Hub Transport server in one of the following ways:

  • Through a Receive connector
  • From the Pickup directory or the Replay directory
  • By direct placement in the Submission queue by the store driver
  • Through agent submission

Every message that's sent or received by an Exchange 2010 client must be categorized on a Hub Transport server before it can be routed and delivered. After a message has been categorized, it's put in a delivery queue for delivery to a mailbox in the same Active Directory site as the Hub Transport server on which the message was categorized or for routing to a recipient in a different Active Directory site or forest, or to a recipient outside the organization.

The Exchange 2010 transport pipeline consists of the following components and processes:

  • SMTP Receive   When messages are received at the Edge Transport server, anti-spam and antivirus agents filter connections and message contents, and help identify the sender and the recipient of a message while the message is being accepted into the organization. When messages are received at a Hub Transport server, transport rules are applied and, if anti-spam and antivirus agents are configured, these agents provide an additional layer of anti-spam and antivirus protection.
    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 into the organization. After a message has passed completely through SMTP Receive and isn't rejected by receive events or by an anti-spam and antivirus 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. There are four types of submission:
    • SMTP submission through a Receive connector.
    • Submission through the Pickup directory or the Replay directory. These directories exist on the Hub Transport server or Edge Transport server. Correctly formatted message files that are copied into the Pickup directory or the Replay directory are put directly into the Submission queue.
    • Submission by the store driver, which picks up messages from a sender’s Outbox as they're sent.
    • Submission by an agent.
    On the Edge Transport server, submission is generally only through the Receive connector. On the Hub Transport server, submission can occur through a Receive connector, Pickup directory, Replay directory, or store driver.
  • Categorizer   The categorizer picks up one message at a time from the Submission queue. On the Edge Transport server, categorization is a short process in which the message is put directly in the delivery queue. From the delivery queue, the message is routed to a computer that's running a Hub Transport server role in the organization.
    On the Hub Transport server, 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. A mailbox delivery queue delivers the message to a local mailbox by using the store driver. A remote delivery queue delivers the message to a remote recipient through a Send connector.
  • Local Delivery   Only messages that are sent to a recipient with a mailbox in the same Active Directory site as the Hub Transport server on which categorization occurred are delivered locally. In this case, local delivery means delivery in the same Active Directory site. All messages delivered locally are picked up from a delivery queue by the store driver and put in the recipient’s inbox on a Mailbox server.
  • SMTP Send   Messages that are sent to recipients in Active Directory sites that differ from the computer that's running a Hub Transport server role on which categorization occurred are delivered remotely or outside the organization. All messages that are sent to a different Active Directory site, to a mailbox that resides on a computer that's running an earlier version of Exchange, or to a mailbox that resides in a different Active Directory forest must be routed through a Send connector to a Hub Transport server that can deliver the message to the intended recipient. All messages that require delivery through the Internet must be routed through a Send connector to an Edge Transport server that can send messages to the Internet for delivery outside the organization.
  • Client Access and Unified Messaging Scenarios   Several client access scenarios and Unified Messaging scenarios don't interact directly with the transport pipeline. Users of Microsoft Outlook 2007, Office Outlook 2003, Outlook Web App, Outlook Voice Access, and Exchange ActiveSync interact directly with the Client Access server role, Unified Messaging server role, and Mailbox server role to access their mailbox. In each case, when mail is sent, the message is put in the sender’s Outbox directly on the Mailbox server by Outlook or the Client Access server on behalf of the sender.
    Aa996349.note(en-us,EXCHG.140).gifNote:
    Outlook Voice Access requires interaction with the Client Access server and with the Mailbox server through the Unified Messaging server.
    After the message is put in the sender’s Outbox, the store driver is alerted by the Microsoft Exchange Mail Submission service, which retrieves the message from the sender’s Outbox, and then puts it into the Submission queue on a Hub Transport server in the same Active Directory site as the mailbox from which the message was retrieved.

The following figure shows the relationships among the components in the Exchange 2010 transport pipeline.

Overview of the transport pipeline in Exchange 2010
Transport pipeline overview diagram

Looking for management tasks related to managing transport servers? See Managing Transport Servers.

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