|Chapter 6 - Managing E-Mail Connectivity|
|Monitoring and Managing Microsoft Exchange 2000 Server|
|by Mike Daugherty|
|Digital Press 2001|
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, Microsoft has continued to focus on Internet standards and has switched from X.400 to Simple Mail Transfer Protocol (SMTP) for all internal (i.e., Exchange-to-Exchange) communications. Even within Exchange routing groups (there are no longer Exchange sites), messages are sent from one system to another using SMTP. RPCs are no longer used for server-to-server communications, even in high-bandwidth situations. (MAPI clients such as Outlook still use RPCs to communicate with the Exchange server.) The high-bandwidth and low-latency requirement for using RPCs was one of the factors that most commonly dictated Exchange 5.5 site boundaries. The bandwidth and latency requirements for SMTP are less stringent and this should provide more freedom for designing Exchange 2000 Routing Groups.
Another significant Exchange 2000 improvement is with the routing engine that now uses dynamically updated link state information about the condition of network and Exchange server resources. The link state status is transferred to all Exchange servers in the organization so that each Exchange server can make an intelligent routing decision based on the most current network conditions.