Science Fair Project Encyclopedia
In computing, X.400 is a OSI standard developed by the ITU-T (at the time the CCITT) and promulgated by ISO for the exchange of email (at the time called IPMs - Inter Personal Messages). Like most ISO standards to do with application-level networking, it never caught on in general use, perhaps due to its complexity compared with SMTP. It is still used in some limited applications -- military/intelligence use and aviation.
X.400 was first published in the 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.
Conceptual protocols were defined for the communication between user agents, even if this would not occur directly, treating P1 and P3 as providing an underlying reliable transport of message contents. The inter-user agent protocol (the message content standard) in X.400 (1984) was named P2, in X.400 (1988) it was named P22.
Important features of X.400 included structured addressing, the possibility of multimedia content (predating MIME), and integrated security capabilities. As X.400 MTAs were assumed to be run by PTTs, X.400 incorporated fields for the automated transfer of messages between X.400 and other 'protocols', such as Telex, facsimile and physical postal mail. Its need for a central control may have something to do with why it never became popular.
- Microsoft Exchange Server: Using Industry Standards for Greater Compatibility - The Use of x.400 in Microsoft Exchange Server 5.5 (Published August 1st 1995)
- Harard Alvestrand's x.400 FAQ - A comprihencive list of resources on x.400 series of standards
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