X.400 is a standardized Message Handling System (MHS) according to the Open Systems Interconnection (OSI). It defines the exchange of electronic messages and is based on the OSI reference model, where it is located in the application layer. X.400 is a generally valid message handling model (MHS).
Structure of the X.400 SystemUsers of X.400 are represented by User Agents (UA) in this model. Intercommunicating Message Transfer Agents (MTA) forward messages from one User Agent to one or more other User Agents using store-and-forward techniquesPrivate Management Domains (PRMD), which consist of the Message Transfer Agents (MTA) with the connected User Agents (UA) or Remote User Agents (RUA).
The basis of the MHS is the Message Transfer System (MTS), which handles the transmission of the various messages in the MHS. The MTS essentially consists of several message transfer agents that route, store, and transport messages through the message transfer system. The task of the User Agent is to construct the message content, package the message in the envelope, and perform receive-side unpackaging and message interpretation. The Message Store forms an intermediate storage for messages that have been delivered by the MTA but could not yet be processed by the receiver UA. The communication between the individual objects is regulated by the P protocolsinformation that is necessary for the MTS to switch and forward the messages.
According to OSI terminology, the PDUs transported by the MTS are called Message Protocol Data Unit (MPDU). Two MPDU types are used, a User MPDU, which is used for user-generated data, and the Service MPDU, which is used for testing purposes or for the transmission of system-relevant information.
Other X.400 variantsThe X.400 specification was first published in 1984 as X.400/84 and was significantly expanded in 1988 as X.400/88 and in 1992 as X.400/92.
For example, in the 1988 standard, the message store feature was added and some protocols were defined much more precisely. In addition, other features such as security and interface to the directory services of X.500 were added.