Spamcop BL: Another Look (It’s Accurate!)

If you know me, you know that in the past, I’ve made no secret of my disdain for the Spamcop DNSBL, aka the SCBL.

I’ve worked in spam prevention, deliverability, and the email realm for a long time, in various capacities. I’ve created and run at least two blacklists that you know about. Later, I helped to design and create a system that processed thousands of confirmed opt-in/double opt-in newsletter signups a day. Combine those two details together and that’s what led me to take issue with Spamcop. My employer’s COI/DOI signup servers kept getting blacklisted by Spamcop, based on some really bad math to measure email volume thresholds and make a determination as to what to list.

I was trying to do the right thing. I was implementing what Spamcop (and other anti-spam groups) want: confirmed opt-in/double opt-in. Yet Spamcop was listing the servers and subsequent mailings regardless. It made me really frustrated, and I was very disappointed. See, it’s not really fighting spam. It’s just blacklisting mail you don’t like, or don’t care about. While perfectly allowed, I am of the opinion that it’s lame to do so under the banner of “fighting the good fight” to stop spam. I’ve shared my thoughts on this topic in just about every available forum—websites, blogs, discussion lists. I know I’ve personally guided many sysadmins away from using the SCBL in the past, because it was easily, demonstrably, listing things that were obviously not spam.

In February 2007, I found that Microsoft was using the SCBL to filter/reject inbound corporate email. (Note that I said corporate email—mail sent to users at micrsoft.com, not users of MSN or Hotmail. I don’t know whether or not SCBL data is used in MSN Hotmail delivery determination, but from what I’ve observed, it doesn’t seem to be.) This started me off on another rant on how ill-advised I felt this was, based on my prior experiences with Spamcop. Some kindly folks (and some less kindly) suggested that I needed to revisit my opinion of the Spamcop blacklist, because things have changed.

After a lot of measuring and discussion, I’m here to tell you: Spamcop’s blacklist has changed, and for the better. It works very well nowadays, as personally measured by me. The open question on Spamcop was what drove me to dive into my massive blacklist tracking project. I started that back on March 10^th. Ever since then I’ve been compiling data on Spamcop blacklist matches against both spam and non-spam. Here’s what I see:

Blacklist	Spam hits	Acc %	Ham hits	Failure Rate
Spamcop SCBL	156194	49.37%	0	0.00%
Spamhaus ZEN	255521	80.77%	5	0.10%
Spamcop+ZEN	267795	84.65%	5	0.10%
Range:	~ 74 days
Total Spam	316348
Total Ham	4999

As you can see, Spamcop helps you attack nearly 50% of spam received, while affecting no legitimate senders. Very few lists do better. Spamhaus ZEN (which combines multiple lists) does better, but will occasionally have a false positive, based on some reputational issue perceived with a given sender.

My recommendation: Spamcop’s blacklist is safe to use, and will effectively help you reduce the amount of spam you have to deal with. Where I find it particularly useful is as an addition to Spamhaus ZEN: If you block mail from entities on either list, you get a 3.8% percent boost in effectiveness. Meaning, just under four percent of my spam hits are found on the Spamcop list, but not on Spamhaus.

For historical reasons only, here are links to my previous articles on Spamcop:

Spamcop Roundup
http://www.dnsbl.com/2007/03/spamcop-roundup.html
Spamcop BL: A blacklist with a hair trigger http://www.dnsbl.com/2007/02/spamcop-bl-list-with-hair-trigger.html
Microsoft using Spamcop BL http://www.spamresource.com/2007/02/microsoft-using-spamcop-and-spamhaus.html
My Problems with Spamcop
http://www.spamresource.com/2003/03/problems-with-spamcop.html