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My company's network has 5000 mailboxes spread across seven different sites. Three of these sites contain mailbox servers with a bridgehead server. We use X.400 connections to link these sites. These three sites connect to a site that is a Microsoft Mail connector, and we've configured these connectors with different costs to create redundancy. (You can see costs in site addressing | routing, and you manually define a cost for each connection between two sites, making a cost tree for all connections.) Another site contains legacy information, a Microsoft Mail connector, and a directory synchronization link for Microsoft Mail. We configured these final site connections with an expected high cost (i.e., at a higher cost than others) because we don't want the Message Transfer Agent (MTA) to route any messages across these links. All the servers were running Exchange Server 5.5 Service Pack 1 (SP1). Figure 1 shows a diagram of our messaging network.
When we upgraded the mail servers to SP2, something strange happened: Despite the fact that we'd configured the legacy site for a high cost and very low performance, the MTA began to route messages from all over the organization to Microsoft Mail. We checked the Gateway Address Routing Table (GWART), and the contents of the routing table appeared OK.
After searching for about a week, I found Microsoft Support Online article Q196665 (http://support.microsoft.com/support/kb/articles/q196/6/65.asp). The document says that in some situations, the MTA can send a message through higher cost routes when multiple connectors are available. In this situation, you must upgrade MTA from version 5.5.2448 (SP2) to 5.5.2623 (SP3). Either install the fix following the instructions in the Support Online article or upgrade to SP3.
You will also want to heed the advice in Microsoft Support Online article Q193894 (http://support.microsoft.com/support/kb/articles/q193/8/94) to tune up X.400 connections on bridgehead servers. The Registry changes that this article describes enable longer timeouts and more threads for the MTA service, which improves performance and avoids backup routes.