The ITU-T standard Message Handling Service Protocol (MHS or MHSP) for e-mail and document exchange, X.400 is a complex gateway protocol that permits disparate e-mail systems to interoperate at a minimal level, over either packet networks or asynchronous dial-up circuits. Created in 1984 and updated in 1988 and again in 1992, X.400 functions at Layer 7, the Application Layer, of the OSI PLP Packet GFI LCI PTI Payload Layer 3 LAPB Frame Flag Address Control Packet Frame Check Sequence Flag Layer 2 X.21bis Bit Stream Bit Stream Layer 1 Reference Model. X.400 gained considerable popularity in Europe, largely for in-house implementations. In the United States, however, the preference developed for the Simple Mail Transfer Protocol (SMTP) of the TCP/IP protocol suite, upon which the Internet is based. Although most carriers do not use X.400 in native implementations, they commonly use it as a gateway protocol for X.400-to-SMTP gateways, particularly for international networks. See also Application Layer, gateway, ITU-T, Layer 7, OSI Reference Model, SMTP, TCP/IP protocol suite, and X Series.
An OSI and ITU standard messaging protocol. It is an application layer protocol (layer 7 in the OSI model). X.400 has been defined to run over various network transports including Ethernet, X.25, TCP/IP and dial-up lines. See messaging protocol, XAPIA and CMC. Following is the format of an X.400 address: /c= /admd= /prmd= /o= /s= /g= X.400 Code Purpose /c= country /admd= administrative management domain (public e-mail service) /prmd= private management domain (inhouse e-mail) /o= organization /s= surname /g= given name