With Tiger Envelopes you can stop snoops from reading your email. Same email client. No extra steps. One click to install on Windows and Linux. Configures Outlook, Thunderbird, and KMail. Supports GPG, PGP, Bouncy Castle encryption.
SPAM-Buster is a Java application for filtering SPAM based on statistical algorithms (currently Bayesian). The platform independant application can be used as graphical end-user client and additional service for mail server.
After the presentation on CeBIT 2004 fair finally available: SpamStop works as a Proxy between your Pop3-Client and the -Provider, it uses a Support Vector Machine to classfy incoming mails and automatically learns classifiers from samples of your mails.
The MFlow project aims to create a modular messaging client in Java. Its architecture should make it as easy as possible to add support for new protocols, as well as maximizing configurability.