SMTP Transport Architecture

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The transport subsystem of Microsoft Exchange Server 2003 is a collection of Component Object Model (COM)-based engines that integrate seamlessly with the Simple Mail Transfer Protocol (SMTP) service of Microsoft Windows 2000 Server and Microsoft Windows Server 2003. Because Exchange Server 2003 must extend the Windows SMTP service with its own components, you can run the Exchange Server 2003 Setup program only on a computer running Windows Server 2003 that has the SMTP service installed. It is important to understand that Exchange components, such as the advanced queuing engine, categorizer, and routing engine, do not only extend the SMTP service, but also replace SMTP components with Exchange-specific components. The extended version of the SMTP service supports:

  • Extended SMTP commands for efficient communication between Exchange servers

  • Integration with Microsoft Active Directory directory service for message categorization and routing

  • Integration with the Exchange store for local delivery

  • Message tracking for analyzing message paths in your Exchange organization

This section discusses the following concepts:

  • SMTP service design   The SMTP service runs in the Inetinfo process and when extended through Exchange event sinks, processes all inbound and outbound messages. When messages pass through the transport subsystem, SMTP makes heavy use of Internet Information Services (IIS) resources. You must understand the SMTP service design to understand how the entire Exchange 2003 transport subsystem works.

  • Advanced queuing engine   The advanced queuing engine is a core component in the SMTP transport subsystem and the main dispatcher of transport events. You must understand the tasks of the advanced queuing engine to understand the interaction between core SMTP transport components and Exchange event sink extensions.

  • Transport components   These components process each message after it is received from a remote host or a messaging client. In Exchange Server 2003, all messages must pass through the SMTP transport subsystem. This includes messages sent through MAPI clients, such as Microsoft Office Outlook 2003, Microsoft Office Outlook Web Access, Distributed Authoring and Versioning (DAV) protocol, X.400, and any Exchange Development Kit (EDK)-based connectors, even if the SMTP protocol is not involved, and even if recipients have their mailboxes in the same mailbox store as the sender. If you stop the SMTP service on a server running Exchange Server 2003, all message transfer and delivery stops on that server. You must understand Exchange Server 2003 message handling to understand how transport event sinks process each message.

  • **Store drivers   **Exchange Server 2003 implements an Exchange store driver so that the SMTP service can obtain outbound messages from the Exchange store and deliver inbound messages to Exchange recipients. You must understand the store driver implementation to identify the physical location of messages as they pass through the transport subsystem.

  • Protocol extensions   Protocol extensions implement Exchange-specific SMTP protocol commands, also called extended SMTP verbs. To understand Exchange-specific SMTP protocol features, you must understand how the protocol extensions are implemented.

  • Logging and message tracking   Protocol logging, event logging, and message tracking are features that you can use to analyze the details of message transfer. You must understand the implementation of these features, especially in troubleshooting situations.

This section assumes that you are familiar with the general concept of message handling in Exchange Server 2003. For more information about message handling, see Message Routing Architecture. This section also assumes that you are familiar with the concepts of configuring virtual SMTP servers, SMTP connectors, and routing group connectors. For more information about these concepts, see the Exchange Server 2003 Transport and Routing Guide.