X.400

Definition - What does X.400 mean?

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..

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Techopedia explains X.400

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:

C: Country name
ADMD: Administration Management Domain
PRMD: Private Management Domain
O: Organization name
OU: Organization Unit name
G: given name
I: Initials
S: surname

An email address in SMTP looks like this: andrew.smith@ourcompany.co.uk. 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:

User Agents (UA): These are the components users interact with to compose, submit and receive email messages.
Message Transfer Agents (MTA): These perform all the routing and delivery of the message.
Message Stores: These actually store the message. This is especially useful where the UA is physically separated by the MTA.