X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as “email”.
The messaging standard with the greatest international impact is called X.4OO. It attempts to lay down standards for all electronic messaging system in the world. Largescale messaging services (such as CompuServe and America OnLine) use the X.4OO specification.
If you send a message to an electronic address over one of these services, the service converts the message to comply with the X.4OO specification, and then sends it on to its destination, where it is converted again, into the message format the receiving program uses to make the message understandable to the receiving party.
As experience was gained, more elaborate systems were proposed. In 1988, CCITT modified X.4OO. However, after a decade of competition, e-mail systems based on RFC 822 are widely used, whereas those based on X.4OO have become obsolete. The reason for RFC 822’s success is not that it is so good, but that X.4OO is so poorly designed and so complex that nobody could implement it well.