The ploinMailFactory is a mailing framework that provides extended features like a templating mechanism, multitenancy and localizaion. Internally it uses the commons-email API from Apache.
Spamato is a spam filter system that combines several anti-spam techniques, from which thousands of users benefit. It is available as an add-on for Thunderbird and Outlook, and as a stand-alone proxy for other email clients. IMAP and POP are supported.
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.
CPO is a tool written in Java that manages newsletters. It's focused on corporate needs (privacy, scalability, bounce count, opt-in & out, newsletter statistics).
Mailbox Sweeper is a program to connect to a mailbox, preview and delete selected messages. Useful for cleaning up mailboxes without downloading all the mails to your computer, e.g. for dialup users who do not want to download that 5MB attachment.
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.
flap is a mail user agent written in pure Java. Its main distinctive feature is mounting message folders (like POP3 or IMAP) into a virtual folder tree, just like on Unix. Other features: macros language (w/ BeanShell) and extensive filter rules.