ISO has established an international standard for electronic mail systems. That standard is X.400. It has a companion standard for directory services (allowing the lookup of names, organizations, etc. over a network) known as X.500.

The X.400 standardization would make an outstanding case study for those interested in decisions by committee. As the new standard progressed, every submitter had to ensure that her pet functionality was represented. Nevertheless, large areas of obvious functionality never made it into the standard, such as facilities for asynchronous access. The X.400 standard was finalized far too late to prevent a raft of commercial vendors from implementing nonstandard systems and seeing their wide deployment.

In the minds of governments worldwide, an ISO standard has the weight of law. Many governments have mandated the use of X.400/X.500 services for their electronic mail systems. This is notwithstanding the fact that many such systems are rather small networks gatewayed to the Internet for long-haul connectivity.

X.400 came into standardization at roughly the same time that the growth of the Internet was causing a huge installed base of SMTP/MIME implementations to be fielded. The PC revolution of the 1980s had already occurred, resulting in many proprietary commercial email systems. In short, X.400 was never widely adopted because it would have required expensive and widespread changes in email systems on a huge scale. That simply never happens.

Contrast that with the changes in the Internet mail system to handle binary attachments to Internet mail. The developers of the MIME specification were extremely careful to keep any enhancements backward compatible to the installed base. An end user may not understand a received message without the appropriate software, but the message will pass through the system to its destination. For those operating non-SMTP mail systems gatewayed to the Internet, the switch to MIME often caused lost or mutilated attachments, but messages themselves were delivered. Although this path has been often criticized, it resulted in an Internet-wide upgrade where none would have been possible otherwise. Individual networks could be upgraded as they saw fit, without losing basic connectivity.

For these reasons, X.400 commercial implementations have tended to be expensive, have proprietary extensions for asynchronous (dial) connectivity, and to come with hefty fees for the use of the smaller X.400 networks. Some governments may have been willing to pay the higher price in order to be standards compliant, but the same cannot be said for many businesses or home users. As of this writing, the total number of SMTP/MIME users in the world dwarfs the number of X.400 users, forgetting for the moment that most X.400 users themselves gateway to the Internet and hence “use” SMTP/MIME in the same sense that many users of proprietary mail systems do.

Still, the X.400 and X.500 standards have some interesting features. Aspects of X.400/X.500 that have utility on the Internet have found their way into the RFCs. In a recent development in directory services, several vendors (including Netscape and Microsoft) have implemented services based on a subset of X.500 known as the Lightweight Directory Access Protocol (LDAP). For more information on LDAP, refer to the RFCs listed in Appendix A, Internet RFCs Relating to Email.