This is a follow up to my previous post. I recently looked up an old version of the PBL and decided to count up how many IPs they were blocking. By my calculations, if we expand CIDR ranges, the PBL blocks nearly 390 million IPs.
Actually, technically, the PBL is a negative case scenario in which ISPs say that mail from this particular IP should not be sending mail, as opposed to "we know that these IPs are spamming."
In any case, out of the total number of IPs available, if you refuse mail from IPs on the PBL you are rejecting mail from about 9% of the total available IP space on the Internet. Now *that* is an impressive statistic. If you use the scaled down number, you are rejecting mail from 18% of the IPs on the internet.
That's some fine work there, Spamhaus.