9.1 SMTP and X.400

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Originally defined in 1982 in RFC 821, SMTP has since been widely used as the de facto standard for email exchange-first within the Internet and then between different corporate bodies. At the beginning of the 1990s, SMTP battled with X.400 to become the generally accepted protocol for email exchange. X.400 had many champions, mostly in Europe and including industry bodies such as the Electronic Mail Association (EMA), but the overwhelming force applied by the Internet juggernaut catapulted the SMTP/MIME combination into the lead position.

The X.400-based MTA is the core of first-generation Exchange servers. The presence of X.400 at the heart of these servers reflects the view in 1992-1995 that corporate messaging systems could really only be built around a solid, well-defined, and comprehensive protocol. When Exchange 4.0 was in development, no one could argue the case that SMTP had reached the same level of development as X.400, which at that stage had evolved through several sets of recommendations and been deployed in many corporations around the world. In comparison to the maturity of X.400, SMTP had not yet evolved to accommodate nontext bodyparts (in essence, binary attachments). In real terms, this meant that X.400 was the only way that people were able to send each other attachments, such as Word documents or Excel spreadsheets, without resorting to complex encoding schemes that required manual intervention. Forcing people to encode a document before attaching it to a message is OK in the technical community, but few office-type users have the patience to go through such a laborious procedure.

The MTA executable (EMSMTA.EXE), the Windows NT service (the Microsoft Exchange Message Transfer Agent), and the X.400 connector collectively formed the backbone of the majority of first-generation Exchange enterprise-scale deployments. The pervasiveness of X.400 within Exchange- even today-is the reason why you cannot delete the X.400 addresses created for every mail-enabled account. Without an X.400 address, the MTA cannot route messages to an Exchange 5.5 mailbox. While X.400 is not a very approachable protocol, there is no doubt that the MTA delivered fast message processing. Even the earliest versions of Exchange running on system configurations now deemed puny were able to accept messages from the Store, assess their destination addresses, and route them very quickly. In addition, you cannot underestimate the importance of the MTA during mixed-mode operation, because it is the primary communications mechanism between Exchange 5.5 and Exchange 2000/2003, unless you decide to deploy SMTP connectors instead.

The speed and throughput of the MTA increased steadily as the first generation of Exchange rolled out. Of course, the added power of the computers and use of symmetric CPUs made their contribution, but the MTA provided the underlying speed. Over three releases, multiple service packs, and many hot fixes the MTA grew in reliability and robustness. However, the rapid evolution of the Internet protocols and the general acceptance of SMTP as the way to accomplish global messaging marked the end for the "X" protocols. In today's Exchange infrastructures, the use of the MTA is limited to dealing with messages sent to older Exchange servers or those destined for other X.400 systems. As companies phase out their use of X.400 in their messaging environments, it follows that the number of messages handled by the MTA is in steady decline, whereas the message volume handled by SMTP grows all the time.

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