At one time X.400 was expected[by whom?] to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-mail. Despite this, it has been widely used within organizations and was a core part of Microsoft Exchange Server until 2006; variants continue to be important in military and aviation contexts.
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) recommendation "Message handling system and service overview".
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, 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.
Exchange Server 2007 does not use the MTA object and the X.400 connector (which must use the MTA) is gone in Exchange Server 2007. There are no longer any X.400 default proxy e-mail addresses in Exchange Server 2007.
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.
An X.400 address is technically referred to as an Originator/Recipient (OR) address. It has two purposes:
Mailbox identification - either the originator or recipient. Global domain identification - where a given mailbox is located.
- 1984 defined an OR address as an X.400 address that identified where the user is located.
- 1988 defines it as a combination of a directory name (distinguished name) and an X.400 address.
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:
1984 had two forms for address formats:
- Form 1: (with 3x variants) - primarily uses ADMD and a subset of other attributes
- Form 2: (with no variants) - identifies users by means of telematic terminal (hardware) addresses.
In the 1988 X.400 Recommendations, four forms of addressing were delineated. The 1984 Form 1, Variant 1 format was renamed as the mnemonic O/R address and the 1984 Form 1, Variant 3 and Form 2 format were combined and renamed the terminal O/R address. New forms introduced were the numeric O/R form (a variation of Form 1, Variant 2) and the postal O/R address.
The unwieldiness of this addressing format is believed by many to be one factor in the lack of success of X.400.
- How the Exchange Server 2007 Core Services Work Together
- A Practical Guide to X.400 Addressing by Roger K Mizumori ISBN: 1-85032-210-4
- A Practical Guide to X.400 Addressing by Roger K Mizumori page 26 ISBN: 1-85032-210-4
- X400 Debate: Addresses are ugly
- RFC 1615 - Migrating from X.400(84) to X.400(88)
- RFC 1649 - Operational Requirements for X.400 Management Domains in the GO-MHS Community
- International Telecommunication Union - Telecommunication Std. Sector - List of ITU-T Recommendations
- International Electrotechnical Commission - About the IEC
- Harald T. Alvestrand's X.400 FAQ - A comprehensive list of resources on x.400 series of standards (last updated in 1995)
X.400 Standards 
X.400 standards are available free from ITU-T
- ITU-T Rec. F.400/X.400 |ISO/IEC 10021-1 Message handling system and service overview
- ITU-T Rec. X.402 |ISO/IEC 10021-2 Message Handling Systems (MHS): Overall architecture
- ITU-T Rec. X.411 |ISO/IEC 10021-4 Message Handling Systems (MHS): Message Transfer System: Abstract Service Definition and Procedures
- ITU-T Rec. X.413 |ISO/IEC 10021-5 Message Handling Systems (MHS): Message store - Abstract service definition
- ITU-T Rec. X.419 |ISO/IEC 10021-6 Message Handling Systems (MHS): Protocol specifications
- ITU-T Rec. X.420 |ISO/IEC 10021-7 Message Handling Systems (MHS): Interpersonal Messaging System
- ITU-T Rec. X.435 |ISO/IEC 10021-9 Message Handling Systems (MHS): Electronic data interchange messaging system
- ITU-T Rec. X.412 |ISO/IEC 10021-10 Message Handling Systems (MHS): MHS routing
- ITU-T Rec. X.404 |ISO/IEC 10021-11 Message Handling Systems (MHS): MHS routing - Guide for messaging systems managers
X.400 Products 
- Logsys - ISOPLEX X.400: Vendor and Developer.
- Compucat Research - Compucat Message eXchange (CMX)[dead link]
- AddOnMail - Editor of X.400 user agent software[dead link]
- Boldon James - Vendor of X.400 user agent software
- Deep-Secure - High assurance boundary protection guards, and Military Messaging clients and servers
- Galadrium - LUNAR X.400 User Agent
- Nexor - MMHS products implementing X.400
- Intelcan - Skycom X.400 User Agent
- Isode Ltd - M-Switch X.400 (MTA) & M-Store X.400 (Message Store)
- Incite Software Solutions, Inc. - Messenger 400 (MTA), military messaging servers and solutions
- NET-TEL - X.400 clients (user agents) and servers (MTA and P7 MS)
- Atos Origin - MXMS X.400 server[dead link]
- Xelas Software - Vendor X.400 Message Handling System
- Microsoft Exchange Server - the core engine of versions 4.0 (the original release) through 5.x were based on an implementation of the X.400 standard. The next version, Exchange 2000, replaced this with an SMTP-based engine. (note, however, that all versions support MTAs to permit connectivity to SMTP, X.400, and other mail systems)
See also 
- Standards organization
- International Telecommunication Union (ITU)
- International Electrotechnical Commission (IEC)
- International Organization for Standardization (ISO)
- Electronic Data Interchange (ANSI Standard)
- Message Handling System (unrelated Novell email protocol)