On Thu, 2007-10-04 at 11:00 -0400, Brandorr wrote:
> All though it is not part of mailman, I have seen spam filters, that
> manage a whitelist, by sending an email back to the user confirming
> that they meant to send the email. http://a-s-k.sourceforge.net/
Unfortunately, for quite some time, spammers have been using
randomly-chosen or maliciously-chosen harvested email addresses in both
the message headers and as the envelope sender (SMTP's MAIL From:
command).
As a result, challenge-based systems usually end up sending messages to
random third parties -- and such messages are themselves spam (you also
see the term "backscatter" used).
A better (but by no means foolproof) method is to reject such messages
during the SMTP exchange with a 5xx-class status code; it appears that a
custom "milter" (real time mail policy filter) could do this so this
reduces to a simple matter of coding.
- Bill
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