Overview

The message transport is the heart of any mission-critical enterprise messaging system. The message transport must be fast enough to support your message traffic, it must be scalable to support growth, and, above all, it must be reliable so that your users will have confidence in the system. I once worked with a company whose departmental messaging system was so unreliable that users routinely followed up all e-mails with a fax because they could never be sure that the e-mail would arrive. Such redundancy obviously affects user productivity and cost. The importance of the message transport is directly related to the distribution of your user population. If you have users in many cities, you must have a reliable message transport.

The Exchange 5.5 message transfer agent (MTA) was based on the international X.400 standard and met all of the requirements of speed, scalability, and reliability. Within an Exchange 5.5 site, Exchange used synchronous remote procedure calls (RPCs) to route messages from one system to another. The RPC-based Site Connector was the most commonly used connector for sending messages between Exchange sites, but other connectors such as the X.400 connector and Dynamic Remote Access Service (DRAS) connector were also available. Connectors (or gateways) were also available to connect the Exchange environment to other messaging systems including Microsoft Mail, Lotus Notes, cc:Mail, and SMTP-based mail systems.

While the X.400-based MTA met all of the requirements, it was out-of-step with the tremendous growth of the Internet and Internet-based protocols. With Exchange 2000,...