More on Spamhaus ZEN

Yesterday I wrote about how Al Iverson had observed zero false positives from using the Spamhaus ZEN blocking list. I also noted that the sample set was too small (~2300 emails) to definitively give Spamhaus the Zero False Positives Seal of Approval.

Today, numbers provided by the Dutch ISP XS4All were brought to my attention.

In a nutshell, before ZEN, XS4All was using a combination of SBL + XBL + dynablock.njabl.org (ZEN is simply a combination of the SBL, the XBL and the PBL). With this combination, they were blocking about 4 million messages a day, out of 8 million examined. Their abuse@ address receives complaints about false positives about once every two weeks. Assuming that only one false positive in a thousand actually generates a complaint, that's a false positive rate of .00089%. Not too shabby.

(Of course, the one-complaint-in-a-thousand number is plucked straight from the air; I have no idea if anybody has ever done a study to find the actual number, but I think it's a reasonably conservative guess.)

The full article (in Dutch) can be found at Vincent Schönau's blog at XS4All.