Apache Hupa is a subproject of the Apache James project, and it is a webmail client developed with GWT, it uses a server side able to get messages from any IMAP enabled server (it uses the javax.mail api).
Although you can use directly Hupa to read and send emails using your email server (there is a properties file to define imap and smtp parameters), in your case you could get the server side to connect to your servers and use any GWT ajax mechanism to get the messages like Hupa client side does.
Here you have a live demo of Hupa.