For someone providing e-mail services, allowing spam through is bad. Go0d mailserver admins get their spam rejection rate as high as they can.

But for someone providing e-mail services, flagging good e-mail as spam, known as a false positive, is really bad. Good mailserver admins have a false positive rate of 0%.

Looking through e-mail bounces from a (legitimate, opt-in) bulk e-mail sender, I’ve discovered a few things that are done wrong. For one, people are just using really bad lists. The five-ten-sg.com blacklist is a notorious example. It took me a long time to get unlisted from them, because someone else in the same datacenter had sent them spam once upon a time. They’re far from the only blacklist doing this, but the point is the same: look into the blacklists you use before you reject mail because of them!

Another thing, though: don’t reject mail because one blacklist says it’s bad. When I get around to setting up a new mailserver, my plan is to score IPs based on how many blacklists they have, weighing more accurate blacklists more heavily. Tools like SpamAssassin do this already. (My plan is to delete from the graylist table when IPs show up in numerous trustworthy blacklists; my area of interest is in the ability to reject mail before they even deliver the message body.)

In other news, my table of IPs that have delivered mail to various spamtraps in the past week have been in overdrive. Just over 2,000 hosts; the most recent 100 all came in within the past 8 hours. The month’s graph is pretty surprising:

The list is available here, but heed my warning above: don’t trust it alone.