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..


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:

  1. C: Country name
  2. ADMD: Administration Management Domain
  3. PRMD: Private Management Domain
  4. O: Organization name
  5. OU: Organization Unit name
  6. G: given name
  7. I: Initials
  8. S: surname

An email address in SMTP looks like this: [email protected]. The equivalent in X.400 would be: G=Andrew, S=smith, O=co, OU= ourcompany and C=uk, so [email protected].
An X.400 setup consists of several components:

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

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Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.