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X.400 is a suite of protocols defining standards for email messaging systems. It was defined by the ITU-TS (International Telecommunications Union—Telecommunications Sector) in 1984 and again in 1988. Used as an alternative to the more common email protocol called Simple Mail Transfer Protocol (SMTP), X.400 is more widely used in Europe and Canada than in the U.S..
X.400 is more complex than SMTP. However, it is familiar to many email server administrators who use Microsoft’s Exchange email server. Exchange also supports SMTP because Exchange is used globally and must support as many standards as possible.
An X.400 address consists of several elements:
An email address in SMTP looks like this: firstname.lastname@example.org. The equivalent in X.400 would be: G=Andrew, S=smith, O=co, OU= ourcompany and C=uk, so GS@OU.O.C.
An X.400 setup consists of several components: