You can configure many e-mail servers to perform a reverse DNS lookup for a host record in the sending server's domain that matches the IP address of the sending server. Validating servers in this way can reduce spamming because the system rejects messages from nonvalidated servers.

Windows 2000's SMTP service, which you manage through the IIS console, offers several advanced delivery options. To view these options, click Advanced on the Delivery tab of the virtual server's properties.

One of these advanced options is Perform Reverse DNS Lookup For Incoming Messages. But this option doesn't do what you might think it does.

Windows 2000's SMTP service doesn't accept or reject connections based on the success or failure of a reverse lookup. Instead, when you enable the reverse lookup option, the SMTP service performs a reverse lookup on the sending server.

If the existence of a PTR record in the target reverse lookup zone verifies the server, the service makes no changes to the message's RECEIVED header. If the reverse lookup fails, the word "unverified" appears after the IP address in the header.

While this reverse DNS option can be useful in identifying messages that were sent from servers with (or without) valid PTR records, it does nothing to curb spam. In addition, enabling reverse lookup will certainly increase network traffic and decrease server performance, particularly if your server receives a large number of messages.