XCON: MTA Routes Messages to EDK Connector when Alternate Route Exists

If an Exchange Development Kit (EDK) Connector is installed on an Exchange Server 5.5 computer, and the EDK Connector has the same address space as another Exchange connector, the Exchange message transfer agent (MTA) will possibly route messages incorrectly to the EDK Connector.

For example, a company develops an EDK Connector that can handle X.400 addressing schemes. This company also has to send X.400 mail out an X.400 Connector. This is not a problem if the address spaces are different. But if the same message handling capability is needed for both connectors, this presents a problem.

On a typical X.400 Connector for an entire organization, the address space is X400:c=*. If both connectors have c=*, it is possible for messages that are meant for the EDK Connector to be routed out the X.400 Connector, resulting in a non-delivery report (NDR). The problem is also manifested by incoming messages from the EDK Connector into Exchange. If the message coming into Exchange is routed right back to the EDK Connector, an NDR will be generated if the message travels out the X.400 Connector.

Microsoft has changed the behavior of the Exchange MTA in Exchange Server version 5.5 Service Pack 2. The MTA now checks to see which connector the message originated from. If there is a duplicate route that is equally weighted in all categories, the MTA tries the duplicate route rather than sending the message back to the originating EDK Connector.

To resolve this problem, obtain the latest service pack for Exchange Server version 5.5. For more information, please see the following article in the Microsoft Knowledge Base:

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.5. This problem was first corrected in Exchange Server 5.5 Service Pack 2.