During the past three years we have been developing an anti-spam application that blocks spam at the level of the receiving mail server even before it reaches the server. The application is targeted specifically against botnet spam, spam that originates primarily from herds of infected Microsoft Windows PCs. It works by blocking the spam at the negotiation phase of each incoming email connection on the basis of DNS (Domain Name Service) information of the sender before accepting email from it.
Most anti-spam systems filter spam rather than block it. This means that the spam reaches your email server, stealing connection bandwidth. The spam then steals an imense amount of processing time, either on the server, the recieving PC or both when it is filtered by traditional anti-spam applications. By blocking spam from reaching the receiving email server we realize a huge saving in both bandwidth and processing time.
Our anti-spam application currently works with the Sendmail and Postfix email servers. We are currently working on versions for other servers as well.