Columba
Columba is an Email Client written in Java, featuring a user-friendly graphical interface with wizards and internationalization support. Its a powerful email management tool with features to enhance your productivity and communication. So, take control of your email before it takes control of you!

Grendel
Grendel is a mail/news reader entirely written in Java. Its goal is to be a true cross-platform application with a feature set that satisfies the poweruser.

ICEMail
ICEMail is an email client written entirely in Java.
It supports the latest email standards, including SMIME, vCard, and PGP, and is based on the Java Mail, Activation, and Help libraries. ICEMail source code and binaries are free to the general public.

Polarbar
The Polarbar Mailer is a 100% pure Java email application that runs on all platforms supporting Java. This software is free to all users and supports most standard email technologies used today. Proven platforms include FreeBSD, Linux, Windows NT/2000, Windows 95/98/ME, Mac OS and OS/2 Warp 4.

Pooka
Pooka is an email client written in Java, using the Javamail API. It supports email through the IMAP (connected and disconnected) and POP3 protocols. Outgoing mail is sent using SMTP.