X.400 Protocol

X.400 was defined by the CCITT (a French acronym for International Telegraph and Telephone Consultative Committee).
This committee is now named ITU-T.
X.400 is the universal protocol for e-mail. It defines the envelope for e-mail messages so all messages match to a standard format.The CCITT recommendations X.400 through X.430 define an Application layer protocol and a minimal Presentation layer protocol.
CCITT X.400 uses the ISO Session Layer services and protocol documented in ISO documents
X.400 comprises three major components:
The User Agent (UA)
The User Agent has immediate interaction with the system.
The user agent allows the user to compose submit and receive mail messages.
The messages that the user receives are not just Inter Personal Messages (IPM) but the user can also receive receipt (and non-receipt) notifications and delivery reports.
The Message Transfer Agent (MTA)
The MTA acts as the message store and the forwarding agent and can also act as the gateway by translating messages into the correct format for the user.
Thus MTA is where the actual transmission of messages take place.
They also perform all the needed routing and delivery functions.
In order to avoid spurious non-delivery problems for the UA the UA had to be available from the MTA at all times.
This problem was solved with the introduction of Message Store (MS).
It is MS which is available to the MTA at all times and supplies a store and forward service to the UA.
This is particularly useful in the more distributed modern environment where the User Agent may be running on a users personal machine and hence might not be available for long periods of time.
The Message Transfer System (MTS)
This provides the messaging infrastructure through which users can communicate.
The MTS comprises a group of MTAs.
The Access Unit (AU)
For the Inter Personnel Messaging Users to communicate with users of pre-X.400 technologies such as telex teletex and even postal delivery services the AU's were introduced. An AU is a device that acts like an interface between MTS and pre-X.400 on the other side. It converts formats service elements and protocols and provides non-X.400 services bi-directional flow of messages.