Reverse Domain Name System Lookups

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If you receive messages directly from other domains on the Internet, you can configure your SMTP virtual server to perform a reverse Domain Name System (DNS) lookup on incoming e-mail messages. This verifies that the Internet Protocol (IP) address and fully qualified domain name (FQDN) of the sender's mail server corresponds to the domain name listed in the message. However, consider the following limitations to reverse DNS lookups:

  • The sender's IP address may not be in the reverse DNS lookup record, or the sending server may have multiple names for the same IP, not all of which may be available from the reverse DNS lookup record.

  • Reverse DNS lookups place an additional load on the Exchange server.

  • Reverse DNS lookups require that the Exchange server is able to contact the reverse lookup zones for the sending domain.

  • Performing reverse DNS lookups on each message can result in a substantial decrease in performance due to increased latency.

Note

For more information about using reverse DNS lookup, see Microsoft Knowledge Base article 319356, "HOW TO: Prevent Unsolicited Commercial E-Mail in Exchange 2000 Server."