X.400 Transport Architecture


Microsoft Exchange Server 2003 uses Simple Mail Transfer Protocol (SMTP) for native message transfer. However, the core components of Exchange Server 2003 include a message transfer agent (MTA) that is also compliant with the X.400 recommendations of the 1988 conformance year. Therefore, you can use X.400 connectors to build the messaging backbone of your Exchange organization or to connect to an external X.400 messaging system. By choosing to use X.400 connectors, rather than SMTP connectors, you add an extra layer of security. This occurs because the X.400 standard requires MTAs to authenticate themselves before the MTAs can transmit messages. Note, however, that X.400 MTAs and X.400 connectors are more difficult to maintain than SMTP connectors. For example, X.400 e-mail addresses are not user-friendly because of their numerous attributes. X.400 is a complex standard that defines the architecture of a message handling system (MHS), based on the following recommendations: X.200, X.217, X.218, X.227, X.228, X.402, X.411, X.413, X.419, X.420, X.435, X.680, X.690, X.880, X.881, and X.882.

The X.400 standard was originally developed in the 1980s by telecom companies, under the umbrella of Consultative Committee for International Telephone and Telegraph (CCITT), which later became the Telecommunications Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T publishes updated X.400 recommendations every four years. Each update introduces new features but remains compatible with previous versions. The first official X.400 recommendation was published in 1984 and is referred to as the Red Book because of the color of its cover. The 1984 X.400 recommendation had several weaknesses in the area of message handling. The 1988 X.400 recommendation introduces additional X.400 message bodyparts and envelope properties. Object identifiers describe message attachments precisely, so that attachment names and other object properties are preserved. The 1988 X.400 standard is referred to as the Blue Book.


The ITU-T uses the letters from the English alphabet to categorize their standards: The "X" in X.400 indicates that the standard is for data networks and open system communications. For details about "X" standards, see the ITU-T Web site (https://www.itu.int/).

This section discusses the following concepts:

  • Exchange MTA in the Exchange Server 2003 architecture   This section explains how Exchange MTA integrates with other Exchange Server 2003 components and provides a general overview of message transfer through X.400 or MAPI-based gateway connectors.

  • X.400 connectors and transport stacks   X.400 message transfer is governed by protocols that determine how MTAs communicate with each other. X.400 connectors and transport stacks define these communication parameters for Exchange MTA. A clear understanding of these protocols and parameters is helpful when configuring connectors and transport stacks. You must understand X.400 communication prerequisites to troubleshoot X.400 connectivity problems.

  • Connecting routing groups using X.400 connectors   When Exchange MTAs communicate with each other through X.400 connectors, they do not necessarily conform to all aspects of X.400. Specifically, Exchange MTAs support message formats that are not defined in X.400. They exchange link state information so that all servers running Exchange Server 2003 in an organization can perform dynamic message routing, as explained in Message Routing Architecture.

  • Exchange MTA in a mixed-mode organizationIf you are running a mixed-mode organization, you must understand the various tasks that the Exchange MTA must perform to support seamless integration of Exchange Server 2003 with Exchange Server 5.5.

This section assumes that you are familiar with the design of message routing topologies and the configuration of X.400 connectors. For more information on X.400 connector configuration, see the Exchange Server 2003 Administration Guide.