Chapter 9: Getting the Mail through-Routing and Message Delivery

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Overview

As a messaging system, the primary task for Exchange is to deliver messages in a reliable and robust manner. The Routing and Transport Engine is responsible for this work. Message routing encompasses three major tasks:

• Server to server message delivery

• Routing between different groups of servers

• External and local message delivery

Exchange has always been good at routing and delivering messages. First-generation servers hold information about all of the other servers in the organization, including how the servers connect together in the Exchange Directory. The X.400-based Message Transfer Agent (MTA) uses this information to route messages quickly and effectively, and most of the time things work well. The wide range of available connectors helps Exchange to link up with its major competitors; many of the connectors also handle directory synchronization, which is of major assistance when you need to migrate from another email system. The combination of comprehensive connectivity and high-fidelity directory synchronization has been the cornerstone of Exchange's success in the enterprise messaging market. On the down side, when things go wrong with the MTA, administrators can tear their hair out to discover where the problem lies. Because of its age and the complexity of its code, Microsoft never upgraded the MTA to be a cluster-aware component.

Exchange 2000 and 2003 still support X.400, but the focus is now firmly on Internet protocols as the foundation for messaging-with SMTP being the most important message transport protocol. As elsewhere in Exchange, the AD is the repository for user email addresses, system information, and the routes served by connectors. Exchange uses a mixture of LDAP and ADSI calls to fetch information from the AD. Unlike other SMTP email systems, Exchange does not use DNS MX records to decide how best to route messages within an Exchange organization; but Exchange does use MX records whenever Exchange needs to send messages to an external SMTP server. The logic here is that MX records simply indicate that a server can accept email, and an Exchange server already knows this fact about other Exchange servers in the same organization.

To some degree, Microsoft took a huge risk when the MTA was superseded by the Routing Engine in Exchange 2000, but the strategy worked, because Exchange is now able to process more messages faster using the SMTP engine than ever before. The X.400 MTA is still present in Exchange 2003 and Exchange can still route messages across the the older set of connectors, but only after the Routing Engine has examined the messages and concluded that they cannot be processed any other way.

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