I don't know if this has been discussed here before. All the whitelisting
discussion I have seen so far was verifying the existance of users.
From what I see from my logs by far the most percentage of spam is from
hosts that are either on dynamic addresses or e.g. the unsecured
workstation of someone in a company that all get abused, either by
having a "not known about" mailserver or proxy server or ...
IMHO a fast and easy to implement strategy would be not to accept
SMTP connections from hosts that haven't clearly marked themselves
"I am a outgoing MAIL Server".
Such marking can be easily done in DNS in the in-addr.arpa zone either
by e.g. setting a TXT record (preferable with a abuse contact) or a MX
record (either a MX record at all or one with a special prio).
This is better than any DNSBL list, because most reverse zones are
maintained at the ISPs and they should probably know what they are
doing.
This setup is easy, cheap, easily deployable for the senders and the
recipients (existing DNSBL modules need only minor tweaking). Transition
is easy, also, one could use the information to add RFC 2822 Headers
on the existance/absence of those records for use with e.g. spamassasin.
Classification is easy, also: you want spam you don't look at these
records, you don't want spam you do.
I know this is not a solution to eliminate spam in total, but it might be
one to eliminate large amounts of it.
Also if an ISP adds one of those records one could set up legal mumbo
jumbo and the customer can't say "it was a newly setup system and we
didn't know it has a mailserver running".
\Maex
--
SpaceNet AG | Joseph-Dollinger-Bogen 14 | Fon: +49 (89) 32356-0
Research & Development | D-80807 Muenchen | Fax: +49 (89) 32356-299
"The security, stability and reliability of a computer system is reciprocally
proportional to the amount of vacuity between the ears of the admin"
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