SpamAssassin (http://spamassassin.apache.org/) is the leading open-source spam filtering tool. It supports a combination of Bayesian filtering, blacklists, and rules based on message content. After training the filter and adding some additional rules it catches 95 - 99% of incoming spam with few false positives.
This project allows for easily integrating Lotus Notes/ Domino with SpamAssassin (you must install SpamAssassin on the mail server first before installing one of the project releases). The installation program copies several design elements to each user's mail database. One agent runs on the Domino mail server before new mail arrives and checks if the incoming mail is spam. If so, it is moved out of the Inbox folder and into the Junk Mail folder. There are additional agents for manually training the Bayesian filter, and for automatically purging old spam.
We have been using it successfully at Axolotl Corporation for several years. However, it is still considered beta-test software and may not be suitable for production use. Other chefs are welcome to join this project and help improve it.