Incidentally, although it may still be the conventional
wisdom in the IETF that DNSBLs don't work and aren't useful,
in the outside world where 95% or more of mail is spam,
they're essential tools to run a mail server. Although there
are indeed lots of stupid DNSBLs, those aren't the ones that
people use, and there are widely used ones that have
vanishingly low false positive rates that let you knock out
most of the spam cheaply so you can afford to do more
expensive filtering on what's left. Spamhaus estimates,
based on the systems that pay for their data feeds, that
there are about 1.4 billion mailboxes whose mail is filtered
using their lists, and they're the biggest but hardly the
only popular high quality DNSBL. It's pretty clear that
there are a lot more mail systems that do use DNSBLs than don't.
As an operator of a large mail domain, I'd like to reiterate John's
comments above. DNSBLs work, are very cost (and computationally)
effective, and are in widespread use.
Regards
Jason
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