[Mimedefang] OT: DNS sanity check

[Jeff Rife]

On 4 Jul 2007 at 20:18, John Rudd wrote:

> Jeff Rife wrote:
> >
> >> However, I don't reject them on that basis alone.  I mark them as spam 
> >> on that basis alone (5 points in SA).  I only reject when the SA score 
> >> is 10+
> > 
> > That's not too bad, then, but I suspect that the number of messages you 
> > reject solely because of that extra 5 points is a lot smaller than you 
> > think it is, and if you take that into consideration, the false 
> > positive rate is quite a bit higher than you suspect.
> > 
> 
> The false positive rate isn't based solely on rejections, it's based on 
> things which triggered the Botnet rule.

You misunderstand.

Look at the messages that are being rejected solely because of the 
extra 5 points you score for bad DNS.  I'm willing to bet that this is 
a very small number of messages.

Second, it's very likely that that in that small number of messages, 
the the number of false positives is rather high, simply because the 
total number of messages is small and the primary reason for rejection 
has a *huge* false positive rate.  As others have shown, AOL (and many 
other large ISPs) will trigger a "bad" DNS hit when you define "bad" in 
such a narrow way as you seem to be.

> This is among an incredibly vocal user base (professors will
> complain at the drop of a hat!).  

And, like most other user bases, they tend to only care about messages 
getting through from people they already have corresponded with.  So, 
you will rarely hear anything about false-positives.

If Joe Google found some articles by one of your professors and 
couldn't send e-mail because your filter blocked it, unless Joe was 
*very* motivated, he'd probably just give up and move on to another set 
of articles (or send from Google, Yahoo, Hotmail, etc., and never even 
mention that he could not get through from CheapMisconfiguredISP.net).


--
Jeff Rife | "In those days Mars was a dreary uninhabitable 
          |  wasteland much like Utah, but unlike Utah, Mars 
          |  was eventually made livable." 
          |         -- Professor Farnsworth, "Futurama"