Chapter 6: Managing E-Mail Connectivity

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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 electronic mail (e-mail) 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 the 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 transport 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 connector, were also available. Connectors (or gateways) were also available to connect the Exchange environment to other messaging systems, including Lotus Notes and Simple Mail Transfer Protocol (SMTP)-based mail systems.

Although 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 2003, Microsoft has continued to focus on Internet standards and uses 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. (Messaging Application Programming Interface [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 provide more freedom for designing Exchange 2003 routing groups.

Another significant Exchange improvement is that the routing engine 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 on the basis of the most current network conditions.

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