POPFile is an open-source spam filter that's available as a free download at http://sourceforge.net/projects/popfile. (The product can also automatically retrieve updates from the Web.) Instead of adding a set of rules and blocked sites to your email client as most spam filters do, POPFile reconfigures your email client to connect to the POPFile relay, which connects directly to your POP-based email server. Because it's implemented as a proxy, POPFile works with virtually all email clients. POPFile is a relatively lightweight application--on my system, it typically consumed only about 9MB of RAM.
POPFile can filter your incoming mail into multiple folders or "buckets." For instance, I set it to filter mail into four buckets: spam, work, personal, and newsletters. In addition, the default POPFile setup provides an unclassified bucket that it uses when it can't decide where to put a given message.
POPFile uses a Bayesian algorithm for spam filtering, so you need to train the filter to make it really effective. Training consists of using the POPFile UI to put incoming mail in the buckets you defined. POPFile then uses the bucket contents to decide how to classify future mail. The more information that's in each bucket, the more accurately POPFile can classify incoming mail. POPFile started doing a pretty good job classifying spam after just a couple hours of training, but for improved accuracy, I continued training for a week. After 2 months, POPFile was 99.53 percent accurate at categorizing my mail. Although POPFile did a great job of categorizing spam, it wasn't problem-free. The product couldn't connect to multiple secure email servers.