X.400

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 email.

At one time, the designers of X.400 were expecting it 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 , 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.

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