Servers
WebMail, DixieMail, Dwarf Mail Server, JAMS, Java Apache Mail Enterprise Server (JAMES), JEmail, Newsletter Dispenser Server, POP3 Proxy, Refer's Java POP3 server, SMTP Proxy
EyeBrowse
Servlet-based, mailing list archive browser, reading standard UNIX mbox format archives and generating HTML dynamically. [Open Source, Apache]
Fundamentals of the JavaMail API
John Zukowski's tutorial at the Java Developer Connection.
IBM POP3 Beans
Beans for receiving and decoding POP3 email, and for the user interface of a mail application. [Open Source, IBM Public License]
IBM SMTP Beans
Beans for sending email using the standard SMTP protocol. [Open Source, IBM Public License]
Java Outlook Connector
Offers library to access Microsoft Outlook application and associated data. [Commercial]
Plasma
Tries to define a framework for mail applications that makes integrating, replacing and modifying of individual modules as easy as possible. [Open Source, LGPL]