JavaMail API, provides a set of classes for mail and
messaging applications. Royalty-free implementations of the
API are now available from Sun for SMTP, POP and IMAP - and
many other mail systems are supported by third-parties. For
more information, visit the official JavaMail page, at
http://java.sun.com/products/javamail/
http://java.sun.com/products/javamail/FAQ.html
Introduction to the Java Mail API
www.javaworld.com/javaworld/jw-06-1999/jw-06-javamail.html
http://www.gamelan.com/journal/techworkshop/
WebMail in Java: Reading E-mail
http://www.gamelan.com/journal/techworkshop/122799_webmail2.html
JMail - Free Java(tm) E-Mail Server
http://www.jps.at/java/tools/mailserver.html
Webmail - A Java mail server with a web interface for reading and sending emails.
http://webmail.wastl.net//
The Java Apache Mail Enterprise Server (a.k.a. Apache James) will be a 100% pure Java server application designed to be a complete and portable enterprise mail engine solution based on currently available open protocols (SMTP, POP3, IMAP).
http://java.apache.org/james/