Claros
Claros Webmail is a fully MIME-compatible pure Java (JSP) implementation of a Web mail application. It is POP3 compatible and does not require a database to run.

GatorMail
GatorMail is a servlet-based Webmail built on the Struts framework. It was originally developed to meet the needs of the University of Florida. Efficient interaction with the mail store along with a low support overhead are the goals of the project.

jwma
Primarily jwma is a WebMail implementation in Java, enabling the user to access, manage and compose email using a standard web browser.

WebMail
WebMail is a www-frontend to IMAP/POP3 mailboxes. You can compare it to the systems Hotmail, YahooMail etc use. It allows a user to access his mailbox via the world wide web and do most things other mail programs allow to do, even if he is not sitting on his own personal computer or corporate LAN.