The art of defending against unwanted e-mail goes beyond simply installing a few antispam programs and updating...
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the profiles. Spam e-mail is a security threat because it denies availability to resources. With a combined attack such as a spam message with a virus attachment, this threat can quickly become very serious and expensive. As with any information security-related issue, a defense-in-depth posture is required.
DNS blacklists are an integral part of any spam-fighting toolkit. The fact that many users on the Internet are using them means that you block a spammer before the first piece of spam even hits you.
By understanding the technology and philosophy behind individual blacklists you'll be able to choose those that suit your needs. Most e-mail servers and antispam software support DNS blacklists, either natively or through third-party add-ons, so there's no reason why they shouldn't be a part of your antispam strategy.
This chapter from the book The Antispam Toolkit, by Paul Wolfe, Charlie Scott and Mike Erwin, explains what DNS blacklists are, how they work and how to implement them on a mail server. The chapter also discusses how to decide which DNS blacklist are the best to use for your organization's circumstances.
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