Buni Meldware Communication Suite is an enterprise groupware product providing an email server, calendar server, and a Web mail client. The Web mail portion is written using Adobe’s Flash and provides a rich cross-browser experience.

The Apache Java Enterprise Mail Server (a.k.a. Apache James) is a 100% pure Java SMTP and POP3 Mail server and NNTP News server. We have designed James to be a complete and portable enterprise mail engine solution based on currently available open protocols. James is also a mail application platform. We have developed a Java API to let you write Java code to process emails that we call the mailet API. A mailet can generate an automatic reply, update a database, prevent spam, build a mes

Scalix Community Edition is a opensource email and calendaring platform. It includes a full version of the Scalix industrial-strength server and AJAX-based Scalix Web Access (SWA), a cross-browser, cross-platform Web client.

Zimbra is an Ajax application suite, consisting of an email client, a calendar, a contact list. Zimbra is a recently new company that was purchased by Yahoo, Inc. in mid-2007. Zimbra documentation