Knife
- Mail and news client, with support for SMTP, IMAP, POP3, Unix Mailbox, and NNTP. [Open Source, GPL]
- http://bluezoo.org/knife/

Grendel
- Project to build a Java mail/news reader. Based at mozilla.org. [Open Source, MPL]
- http://wiki.mozilla.org/Grendel

JVMail
- Freeware Java SMTP command-line client.
- http://www.quacknews.org/index.php?JVMail_Project

Mailpuccino
- Free, fully-featured email client written as a Java application: Supports POP3 and SMTP, and has an address book, features to view/send/save attachments, delayed posting, etc. [Open Source, GPL]
- http://www.kingkongs.org/mailpuccino/

ZOE
- Indexes and configures e-mail searches.
- http://zoe.nu/

Pooka
- A JavaMail based email client. Supports IMAP (including disconnected mode). Requires Java 2. [Open Source, GPL]
- http://www.suberic.net/pooka/

CraftMail
- JavaMail Client [Open Source, GPL]
- http://www2s.biglobe.ne.jp/~dat/java/applications/CraftMail/index_en.html

ICEMail
- An email client written in Java and based on the Java Mail API. [Open Source, GPL]
- http://www.trustice.com/java/icemail/

HotJava Views
- A set of tightly integrated corporate communication tools for email, calendar management, name directory and web browsing. Supports Windows, JavaOS, and Solaris. [Commercial]
- http://java.sun.com/products/hotjavaviews/index.html