Reducing spam with OpenBSD and spamd

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.