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X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as "e-mail". While X.400 never achieved the universal presence of Internet e-mail, it has seen use within organizations, and as part of proprietary e-mail products such as Microsoft Exchange.



The first X.400 Recommendations were published in 1984 (Red Book), and a substantially revised version was published in 1988 (Blue Book). New features were added in 1992 (White Book) and subsequent updates. Although X.400 was originally designed to run over the OSI Transport service, an adaptation to allow operation over TCP/IP, RFC 1006, has become the most popular way to run X.400.

Developed in cooperation with the ISO, the X.400-series recommendations specify OSI standard protocols for exchanging and addressing electronic messages. The companion F.400-series of recommendations define Message Handling Services built on Message Handling Systems (MHS), as well as access to and from the MHS for public services. In the late 1990s the ITU-T consolidated recommendations F.400 and X.400 and published the ITU-T F.400/X.400 (06/1999) Message handling system and service overview recommendation.

The X.400-series recommendations define the technical aspects of the MHS: ITU-T Rec. X.402 | (ISO/IEC 10021-2) defines the overall system architecture of an MHS, ITU-T Rec. X.411 | (ISO/IEC 10021-4) defines the Message Transfer Service (MTS) and its functional component the Message Transfer Agent (MTA), and ITU-T Rec. X.413 | (ISO/IEC 10021-5) defines the Message Store. All ITU-T recommendations provide specific terms for descriptions of system entities and procedures. For example, messages (email) exchanged among people is referred to as Interpersonal Messaging (IPM); electronically structured business documents (e.g., invoices, purchase orders, dispatch advice, etc.) exchanged among trading partners’ computers fall under the EDI protocols.

As with most ISO standards dealing with application-level networking, X.400 failed to compete successfully with SMTP, the Internet-based equivalent in North America. However, in Europe, South America, and Asia, X.400 is quite widely implemented[citation needed], especially for EDI services. In North America X.400 is still used in some applications, such as the military, intelligence services and aviation, mainly because the X.400 functions for integrity and security were developed and deployed much earlier than their SMTP counterparts (S/MIME, PGP and SMTP-TLS). For similar reasons it is sometimes used for transmission of EDI messages between applications.

Message handling is a distributed information processing task that integrates two related subtasks: message transfer and message storage. The ITU-T Recommendations define specific protocols for a wide-range of communication tasks. For example, the P1 protocol is used explicitly for communication among MTAs, P3 between the user agent and an MTA, and P7 between the user agent and message store.

In the 1994 version P7 was enhanced to provide folders in the message store, allow storage of submitted messages, and provide many automatic actions such as auto-foldering and correlation of replies, delivery reports and receipt notifications with submitted messages.

X.400 message content standards are defined for communication between user agents. These are modelled as conceptual protocols that treat P1 and P3/P7 as providing an underlying reliable transport of message contents. The message content standard for interpersonal messaging, IPM, defined in ITU-T Rec. X.420 | ISO/IEC 10021-7 was named P2 in the Red Book. The extended version of IPM in the Blue Book was given content-type 22 (for P2 version 2) and is often referred to informally as P22, although that term is not used in the standards. The message content standard for EDI is defined in ITU-T Rec. F.435 | ISO/IEC 10021-8 and ITU-T Rec. X.435 | ISO/IEC 10021-9, and informally referred to as P35. A voice messaging content type is defined in ITU-T Recs. F.440 and X.440.

Important features of X.400 include structured addressing, ASN.1 binary code enabling multimedia content (predating and more efficient than MIME), and integrated security capabilities. As X.400 inter-domain relay services were assumed by ITU to be run by PTTs, X.400 incorporated fields for the automated transfer of messages between X.400 and other PTT services, such as Telex, facsimile and physical postal mail. ISO later added open routing standards (ITU-T Rec. X.412 | ISO/IEC 10021-10 and ITU-T Rec. X.404 | ISO/IEC 10021-11), but the initial misconception that X.400 required PTT relay services, coupled with PTT volume-based charges for these, were factors that inhibited the widespread uptake of X.400.

X.400 has been extended for use in military applications (see MMHS) and aviation (see AMHS).


An X.400 address consists of several elements, including:

  • C (Country name)
  • ADMD (Administration Management Domain), usually a public mail service provider
  • PRMD (Private Management Domain)
  • O (Organization name)
  • OU (Organizational Unit Names)
  • G (Given name)
  • I (Initials)
  • S (Surname)

The standards themselves originally did not specify how these email addresses should be written (for instance on a business card); RFC 1685 specified one encoding, based on a 1993 draft of ITU-T Recommendation F.401 which looked like "G=Harald;S=Alvestrand;O=Uninett;P=Uninett;A=;C=no" — the unwieldiness of this addressing format is believed by many to be one factor in the lack of success of X.400.[1]

See also



External links


X.400 Standards

X.400 standards are available free from ITU-T

X.400 Products

  • Logsys - ISOPLEX X.400: Vendor and Developer.
  • Compucat Research - Compucat Message eXchange (CMX)
  • AddOnMail - Editor of X.400 user agent software
  • Boldon James - Vendor of X.400 user agent software
  • Clearswift - High assurance boundary protection guards, and Military Messaging clients and servers
  • Isode Ltd - M-Switch X.400 & M-Store X.400
  • NET-TEL - X.400 clients (user agents) and servers (MTA and P7 MS)
  • Atos Origin - MXMS X.400 server

See also

Further reading

  • Betanov, Cemil (1993). Introduction to X.400. Boston: Artech House. ISBN 0-89006-597-7. 
  • Radicati, Sara (1992). Electronic Mail: An Introduction to the X.400 Message Handling Standards. Mcgraw-Hill. ISBN 0070511047. 
  • Rhoton, John (1997). X.400 and SMTP. Elsevier. ISBN 155558165X. 


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