The Destiny of Blacklists

June 2005

In 1997, a group of anti-spam vigilantes called MAPS started a blacklist of mail servers owned by or compromised by spammers. Mail server administrators could use this list to block sources of spam. At least, that was what most of them thought they were getting.

The problem was, as vigilantes so often do, the guys at MAPS got carried away. They started to include servers on the list that they knew weren't sources of spam, to pressure whoever owned the server to do what they wanted. For example, in order to get revenge on people they believed were spamming, MAPS would blacklist the mail server of the company hosting their site.

MAPS knew these mail servers weren't spam sources. But they'd blacklist them anyway. Everyone else sharing that server would then have their mail blocked. And MAPS could insist that the hosting company delete the site of the (supposed) spammer as the price of all the ISP's other, innocent, users having their mail unblocked.

This is, strictly speaking, terrorism: harming innnocent people as a way to pressure some central authority into doing what you want.

The innocent people whose mail got blocked as a result of this kind of trick weren't "collateral damage." They weren't harmed by accident. It was in order to harm these innocent people, and thus put pressure on their ISP, that MAPS blacklisted them.

This kind of tactic gradually brought MAPS into disrepute. Most mail server administrators dropped their list and switched to another blacklist, the Spamhaus SBL, which was created specifically to avoid MAPS-style abuses. They were only going to list real spammers. And for a couple years they did.

Unfortunately, as so often happens, power corrupted them. About a year ago, I started to hear reports that Spamhaus was starting to use the same tactics MAPS had.

John Reid of Spamhaus told me this wasn't true-- that the SBL was still clean, and that they only blacklisted hosting companies' mail servers when they were spam hosts who took on innocent users as camouflage:
The sad fact is, some of these "spammer friendly hosts" will also try load up with as many non-spammers as they can to try and show legitimacy. We try at all costs to avoid listing legit places and people, and only if the host tells us or shows us in no uncertain terms that they don't plan to cease hosting spammers will we list them in their entirety.
I wanted to believe him. But before I could reply to his mail, I got first-hand evidence that the SBL has in fact gone bad.

As of this writing, any filter relying on the SBL is now marking email with the url "" as spam. Why? Because the guys at the SBL want to pressure Yahoo, where is hosted, to delete the site of a company they believe is spamming.

This clearly contradicts what John Reid wrote in his email to me. Yahoo is not a "spam friendly" ISP that takes on a few innocent users to "show legitimacy." And Spamhaus knows it. Of the tens of thousands of sites Yahoo hosts, how many do they claim have spammed? Two.

This case illustrates an important failing of blacklists. Unlike filters, they're run by humans. And humans are all too likely to abuse the kind of power that blacklists embody. Perhaps someone will start another blacklist that tries to avoid such abuses. But how long before that one becomes corrupt too?

No doubt this particular case will get sorted out, and mail containing my url will stop getting blocked. But this example is enough to prove that the whole idea of blacklists is broken. Blacklists have a structural flaw: there is no one to watch the watchers.

Clarification: Many people seem to assume that Spamhaus merely blacklisted the IP address of a single spammer's site. In fact, as well as the spammer's IP address they also blacklisted, aka, which is shared by thousands of Yahoo stores.

SBL Going Bad?

Another SBL Story

SBL Listing Criteria

An Incorruptible Blacklist?