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The set of ITU-T communications standards covering electronic mail services provided by data networks. X.400 was widely used in Europe and Canada.

X.400 addresses tend to be much longer than RFC 822 ones. They consist of a set of bindings for country (c), administrative domain (a), primary management domain (p), surname (s), given name (g).

For example, the X.400 address,

c=gb;a=attmail;p=Universal Export;s=Bond;g=James;

might be equivalent to RFC 822



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.400Code     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