surely we in the IETF should be able to do better than to have our mail
servers filter incoming mail based on completely irrelevant criteria
like whether a PTR lookup succeeds!
Spam filtering is sort of like chemotherapy, the difference between
the good and the bad is pretty small, and the trick is to find
measures that will kill the disease without killing the patient. It's
entirely a matter of statistics, not fundamental design.
And sort of not like it at all. For one thing, the mutation rate in the spam
world is much higher. And that brings us to the real issue here: Whether or not
a given technique, which was very effective indeed at one point, is effective
enough now to continue using.
I can assure you that in the outside world, the amount of legitimate
mail that comes from no-PTR hosts these days is infinitesimal.
And in my experience the amount of spam coming from such sources, while much
greater, is now distinctly in the minority in terms of all incoming spam. More
specifically, since rather than rejecting mail coming from IPs with no PTR
entry I instead use this as a weighted factor in computing an overall spam
score, I was able to examine today's spam to see to what extent this check is
helping or hurting. Turns out it's doing neither - I could eliminate it
entirely and my false negative rate would not go up by even a single message.
And right now the weight is sufficiently low that it isn't causing any false
This defense definitely was very effective once upon a time, but the world has
adapted. In this particular case what seems to have happened is that because of
various PTR record checks a lot of ISPs now unconditionally assign PTR records
to every address, e.g., cpe-76-171-209-109.socal.res.rr.com and
173.sub-75-217-87.myvzw.com were associated with the last two dynamic IPs I've
used. A few years ago it was common to get assigned IP with no PTR records at
all, nowdays much less so.
And this has now led to a new issue: We now have various systems out there that
are now attempting to parse PTR results order to try and figure out the
difference between static business addresses and dynamic residential addresses.
And as you might expect, they often get it wrong.
Now, of course one advantage of this check is that it can be done before you
even accept the message. But the tradeoff then is that this is giving it
It's one of the filtering rules with the lowest false positive rates.
Other than temporary glitches like the 6-to-4 one, the only place
where I see problems is from auld pharts like us whose mail systems
have been working just fine since the 1980s, and who out of a weird
sort of principle refuse to make changes to bring them in line with
modern practice, even changes that are compatible with equally ancient
So, yeah, spam stinks, but it's not going away, and arguments that you
shouldn't use a technique today because it didn't work in 1998 don't
Nor does continuing to use a technique that has outlived it's usefulness. IMO
the utility of this particular check peaked a few years ago and we're now on
the down-slope in terms of utility.
And in the specific case of the IETF, where we want to encourage people to use
our servers to experiment with IPv6 and where there are bound to be a lot of
PTR record issues, I think an absolute block on the basis of no PTR record is
totally inappropriate unless it can be shown to be singularly effective in
dealing with spam and that our spam would increase dramatically without the
rule. I doubt very much this can be demonstrated.
OTOH, using it as a component in computing an overall spam score may still make
sense, especially if the use of IPv6 could also be factored in.
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