Pierre BRUA wrote:
For your information, this is already used at the monkeys.com domain,
and they say it works pretty well to jugulate spam.
See
http://www.monkeys.com/anti-spam/filtering/additions.html
Read a little further. It only works reliabily enough to be worthwhile
for a very limited number of very large domains. Even if this was RFC'd
into some sort of DNS convention or extension, it's trivially bypassed
by the spammer choosing envelope senders that don't participate in the
convention. Given that, for example, huge chunks of the internet don't
have [r]DNS at all, it's pretty clear it'd never become widespread
enough to be long-term useful.
In other words, anything that relies on a significant fraction of the
non-spammers to do something is doomed to failure.
Yes, the technique can be useful sometimes in specific situations. But
not well enough to enshrine into long-term standards, and useless the
day after publication.
There's a similar heuristic, which is probably more effective in
blocking spam - that is, for selected domains, _insist_ that the
argument to HELO matches the peer's rDNS domain name in some fashion.
But again, while it's currently useful for some specific sites (eg: AOL,
Hotmail/MSN, Yahoo), it's too easily bypassable and compliance will
never be wide spread enough to enshrine into public standards.
It's rather a lot like discussing filtering based upon the phrase
"Diplomas from prestigious non-*ccredited Universities". _Still_ works
well (which is why I munged it slightly). But not for long after
publishing it. Oops ;-)
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