I don't know what you think about webmail in opensource, but I actually not very happy with existing ones (except the Scalix product, but it's not fully opensource). Yesterday, I discover a new webmail (written in Java): Claros inTouch. It has been released at the begin of June 2007, but it works very good.

Some features :

• Support both IMAP / POP3 protocols
• Use a database (mysql only, but should be easy to use another one) to store users preferences, contacts, and more
• Use AJAX for some nice features: mails drag'n'drop beetween IMAP folders
• Support Jabber, users may chat inside the webmail !

Claros inTouch is war (Web Application Archive) which should work on all java application server, I deployed in glassfish w/o any problem. Just think to read and edit the <deploy dir>/WEB-INF/config/config.xml file, especially if you don't want let mysql root access w/o authentication. In this file, there is also some configuration related to the IMAP server, for example, if you use cyrus (like me), you must specify the namespace.

My own screen shot :