The X.400 standards

This page is intended to be part of my X.400 FAQ.

The X.400 standards are published in cooperation by ISO and the ITU.
ISO calls them ISO/IEC 10021, parts 1 to 9; the X.400 name is the only one you need to care about, since it is the only one people remember.
Within ISO, work is done by ISO/IEC JTC1/SC18, "Document processing and related communication", working group 4.

The main variants of X.400 are:

X.400 is not a single standard, but a number of them, ranging from X.400 to X.440.

The complete list of standards in force in the X series can be fetched from the ITU Web listing listing at

The currently relevant X.400 standards are:

The real protocol specifications are X.411, X.413, X.419 and X.420. The rest are add-ons, supporting material, or irrelevant.

Totally irrelevant X-numbers include:

In addition, X.480-X.485 specify PICS proformas for the protocols.

The standards are NOT available to the general public online; for some reason, ISO and the ITU expect to earn money from sales of paper copies of the standards. ITU also sells electronic versions.

Some sites, like ULB, have copies of the base standards online, but these are intended for specific purposes.

Other relevant standards

There are a number of other standards that influence X.400 in practice.

These include:

Some other sites to go look for links to interesting docs:

Last modified: Aug 23, 1998