Linux.com runs a good introduction to spamd, the spam fighting daemon which ships by default with OpenBSD.
Linux.com | Reducing spam with OpenBSD and spamd through Undeadly.org
We all know about the rampant spam email problem. Nearly all of the potential solutions offered for it are based on the idea of the mail server receiving messages, classifying them as either spam or legitimate, and then processing further (deleting or forwarding messages) as appropriate. The problem with this strategy is that you end up using extra resources on the mail server. Here’s a way to get the same result while minimizing resource usage by preventing the spam from reaching the mail server.
This article doesn’t go in the details nor installation (this is going to be in the next article) but it does explain the concepts behind spamd, such as
- greylisting : all new SMTP servers first get into a quarantine list, before eventually reaching the whitelist and be able to deliver emails
- blacklisting : based on known spam servers lists
- tarpitting : all black listed SMTP servers get send to the “tarpit, which is a fake SMTP server with poor TCP parameters, which makes the connection long and painful for the spammers
As explained in the article, spamd is different from other spam solutions (such as spam assassin), because it acts even before the spammer is able to deliver his spam, which makes is lightweight and makes it possible to be put in front of real SMTP servers.