US DMS is an implementation. Few information about this technology is freely available inside US, and especially outside US because of international filtered access to the US .mil web sites.

The public US DMS .mil site (from US). Tech target white paper.

The DMS procurement was granted in 1995 and is primarily intended to replace the DoD's aging Automatic Digital Network (AUTODIN) by using commercial, off-the-shelf components. The national intelligence community also plans to use DMS and other countries' defense agencies are deploying similar messaging systems. Deployments typically start with commercial versions of DMS-capable products, with the DMS components to be added later as they become available. Products supporting DMS are "enhanced" versions of several vendors' X.400 and X.500 messaging products. DMS versions of software are available from Lotus (IBM) in Notes, Microsoft's Exchange 5.5 (native) and later versions with connectors, EXM Mail from Enterprise Security, Ltd., and in Novell's Groupwise. Because the unique for directories requirements are less strict for X.500 directories than message clients and servers, most X.500 product vendors participate in this market segment.

Extract from an article of Victor S. Wheatman of Gartner group

The DMS Message Security Protocol (MSP) was created by the National Security Agency. DMS is best described as an X.400-like protocol which can be used over Transmission Control Protocol/Internet Protocol (TCP/IP) networks. But it is not intended for use over the Internet; it is to be run over the Defense Information Systems Network (DISN). Connectivity between DMS and other messaging systems is to be handled through gateways called Multifunction Interpreters (MFIs).

DMS will initially handle messages up to the "sensitive but unclassified" category and will not be trusted to handle "Top Secret" messages in the short term. Rather, three tiers of messaging will need to be supported: 1) "Unclassified", using the NIPRnet (Unclassified but Sensitive IP router network, that replaces MILNET), which is an subset of the Internet, 2) "Secret" using SIPRnet (the U.S. Government's Secret IP Router Network), and 3) "Top Secret" using the Joint Worldwide Intelligence Communications System. With these three tiers, it is likely that facility and battlefield messaging will require three separate systems. Regardless, every message sent through DMS will be encrypted, hashed, and signed, even if unclassified since DoD considers all e-mail and messaging to be "Sensitive" 

Most of the largest nations, and several smallest uses X400 as a military message handling system, being a basis for Control, Command and Communication centers. Most infomation is classified and therefore unavailable.