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DNS blacklists are used by big Internet Service Providers and independent users alike to determine if incoming e-mail is from a spammer or spam-friendly server. They're also used to list the addresses of servers that may have been compromised by spammers as third-party spam relays.
Blacklists have a sometimes dicey reputation among e-mail administrators, however, for a couple reasons. First, they're not always accurate. Some innocent parties get blacklisted along with the guilty. Second, they're fickle. A blacklist server you were using a week ago may get shut down for no reason other than the person running it decided not to bother anymore.
The first issue is an inherent one with blacklists, and for that reason they should be used as one element among many when dealing with spam. The second issue, fortunately, can be dealt with a lot more directly thanks to a utility called BL-Monitor.
BL-Monitor, provided free by Computer Mail Services, queries a customizable list of DNS blacklist servers at regular intervals and returns detailed, graphically formatted statistics about how well each blacklist server is performing.
The resulting data is ranked according to various criteria -- number of addresses blacklisted, average response time, and so on. From this, it becomes easier to determine which blacklist servers are the most responsive, are returning the most useful information, or both.
BL-Monitor does not require the presence of an e-mail server of any kind; the only requirement is the .NET Framework, 1.1 or greater, and an Internet connection. This is both an advantage and a disadvantage, since it doesn't use IP addresses from live e-mail traffic to do its testing.
The IP addresses used to test the blacklists can be supplied in a plain text file, but this shouldn't be difficult to extract from existing logs. (Computer Mail Services also has a separate e-mail management product called Praetor that can feed IPs directly to BL-Monitor, but most people will probably want to use BL-Monitor in a standalone context.)
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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