The Blog of Harald

Spam volumes have been rising continually around here. I started my foray into automated spam filtering a couple of years back; at the time, I was receiving about 100 per _quarter_. Now I’m getting almost 100 per _day_.

I needed an excuse to upgrade my “postfix”:http://www.postfix.org/ install to the new “2.1 release”:ftp://ftp.utoronto.ca/mirror/packages/postfix/index.html, so I decided to install “postgrey”:http://isg.ee.ethz.ch/tools/postgrey/, a “greylisting”:http://projects.puremagic.com/greylisting/ daemon. So far I’m using it after all of my other spamtraps, but it seems to be working reasonably well. I’ll be watching the logs for a while to make sure…

In a nutshell, greylisting relies on the fact that spammers use dump-and-run tactics, while legitimate email gets queued at the sender. So, when a new, previously unknown client connects, the mailserver sends a “temporary deny”. If that connection is a spammer, they’ll probably not return; the reject means the spam was refused. If the sender was legitimate, it will retry, and our server will allow the retry through.

Pretty cool, if you ask me :-)