DNSbased Blackhole Lists

A discussion about SPAM filtering wouldn't be complete without mentioning DNS-based Blackhole Lists (DNSBLs, also known as Real-Time Blackhole Lists or RBL).

DNSBLs are easily retrievable (usually over DNS or HTTP) lists of IP addresses belonging to supposedly known spammers that can be used for blacklisting in your MTA. They are one of the most widely adopted mechanisms for preventing SPAM. Published by third-party entities using community-submitted entries, they do not provide any guarantee about the published results.

Most of the time DNSBLs do not use a warning header in processed messages but instead reject the sender immediately during the SMTP conversation. Here's a DNSBL effect example:

<Verbatim listing 9> $ telnet mail.example.com 25 Trying Connected to mail.example.org. Escape character is

220 box.example.org ESMTP mail.example.org ; Wed, 11 Oct 2006 12:22:07 -0400 mail from:[email protected] 550 5.0.0 Banned due to spam </Verbatim listing>

Our discussion of DNSBLs will be rather radical: Don't use them. Contrary to popular belief, they are often inaccurate, unmaintained, and prone to false positives. Removing false positives on most lists is a painful process that wastes mail administrators' time, and detecting false positives in the first place is a difficult task.

If implemented, addresses like [email protected], [email protected], and [email protected] should always be exempt to DNSBL checking to allow the banned client to contact the administrator in case of wrong inclusion. Sadly this doesn't happen often.

Many resources are available on the Internet for checking if an address is included in a DNSBL. One of the most accurate is http://www.rbls.org.

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