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Page 15: DNS blacklists
Similar to a reverse DNS lookup to find the name corresponding to an
IP address (e.g. 188.8.131.52 -> www.cam.ac.uk).
We already use the Janet/MAPS RBL+ to reject some email attempts
before we even see the message. This lists open SMTP relays, open web
proxies, dial-up address ranges, and spam-friendly ISPs.
Some lists are more accurate than others so it's not sensible to block
using all of them, but they can be used as a contribution to the spam
score of a message without having such a bad effect on legitimate email.
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