XCON: IMS Queues Back Up to Hosts with Multiple MX Records

If you are running Microsoft Exchange Server 5.0 with the Internet Mail Service (IMS) installed, your server may experience slow delivery to hosts that have multiple MX records of equal weight. Internet hosts supporting multiple IP addresses are often the same hosts receiving high volumes of mail.

When host addresses are resolved by Domain Name Service (DNS), a list of records is returned if the site has multiple MX records defined. The IMS attempts to randomize its choice of the available MX records. This is done to spread the load evenly over the sites' MX hosts. The randomization code is behaves poorly for sites with more than a few MX records of equal weight.

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.0. This problem was corrected in the latest Microsoft Exchange 5.0 Server U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):