Blocking based on rDNS, IMO, is bad -- it punishes an ISP's *users* rather
than the ISP themselves.
For example, until last week, I was using an ISP that did not have rDNS
for their outgoing SMTP servers. Now *I* know the importance of rDNS for
SMTP servers etc., but I was just a user. My 3 support requests didn't
count for much, and as far as I know the servers still have no rDNS.
The stock response to this is "well, change ISP" -- but that's a naive
view of most of the world, where the choice is not available. In my case
(in Ireland), accessing the net through this ISP was 2/3 the price of its
nearest competitor.
No way was I going to pay extra for the privilege of not getting the
occasional bounce from an over-aggressive mail filter. I mean,
considering I'm adminning a couple of spamfilter discussion groups, I get
a bounce message (generally on the level of "your mail contained the word
'viagra'") every 3 minutes anyway ;)
Also, requiring rDNS is not even that effective a test; our rule for this
condition in SpamAssassin gets this rate:
35.5901 3.0576 0.921 0.78 1.33 NO_DNS_FOR_FROM
in other words 35.59% of spam has no rDNS, but 3.05% of ALL nonspam mail
does too. The latter is bad news, it's a *very* high false positive rate.
--j.
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