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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, perhaps due to its complexity compared with SMTP.

X.400 was first published in the 1984 (Red Book), and a substantally revised version was published in 1988 (Blue Book). New features were added in 1992 (White Book) and subsequent updates.

The main protocols of X.400 were: P1 for communication between MTAs, P3 between the user agent and MTA, and P7 between the user agent and message store.

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.

X.400 has been extended for use in military applications.

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