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A useful but undocumented feature of the Internet Mail Service (IMS) in Exchange Server 5.5 is that you can specify multiple hosts for load balancing. My company has multiple SMTP relay hosts, so I decided to use them for load balancing and redundancy. On the Connections tab of the Internet Mail Service Properties dialog box, I specified multiple hosts, separated by commas, in the Forward all messages to host text box in the Message Delivery settings group. This entry tells the IMS to deliver the first message to the first host, the second message to the second host, the third message to the first host (because I specified only two hosts), and so on. This sequence, called a round-robin process, adds redundancy and load balancing for outbound Internet mail.
To achieve the same goal for inbound traffic, I configured the SMTP relays (which are UNIX boxes with Sendmail running on them) to round-robin Internet messages to the IMS servers. To accomplish this configuration, I used a domain name with two DNS MX records with different preference numbers in the DNS for IMS servers. Instead of relaying messages to the IMS servers' IP addresses, I pointed them to the DNS domain name. With this configuration, the organization has quicker Internet message flow. In case one IMS server or SMTP relay host goes down, the company still has uninterrupted Internet mail.