How to buy AntiSpam? Part 2

What is False Positive rate of an AntiSpam software?

False positive is the percentage of legit emails blocked by an antispam compared to the total number of blocked messages. In other words it is a measure that shows how effective a given antispam engine is in differentiating spam from normal messages.

False positive rate is inseparably connected with catch rate. However having a very high catch rate means nothing without low false positive rate, and vice versa. Every antispam can block 100% of spam simply by blocking every message that comes, but to do it without eliminating any legitimate message is what we want.

What is it worth for an AntiSpam engine?

In general, the smaller the false positive rate, the better AntiSpam you’ve got. It is hard to say what false positive rate is good enough, basically it should be somewhere around 0,04%, and everything below that value should work. You can try to estimate the value of this factor as I suggested with catch rate, but this may only give a very general impression.