Well, that is an easy question. The answer is 3.2%.

The “Real reverse DNS test”—that is, checking whether the sending server has a DNS name—is in the Top 10 requested features since years. So far we resisted adding it to ORF, due to the high false positive rate and once or twice a year I ran a test on our legitimate email sample body to see whether the false positive figures changed. My last test was performed on ORF tech support emails from the last 45 days and the false positive rate was an unacceptably high 3.2%.

Sure, we can treat this type of test a policy enforcement tool instead of a spam filter, but come on, who want to lose 3.2% or even 0,5% of the legitimate emails? Yes, we could add a big flashing warning sign telling “You will shoot yourself in the foot”, but I think it is better not to allow the user to shot herself in the foot at the first place.

What do you say?