which webmail software do you use? But generally its not too hard, depending on the loginprocess. Maybe you just use sth. like httpwach (plugin for msie) to see which data is sent to the server (or set on the client when accessing the loginpage). These request is what you have to simulate in order to e.g. perform the loginprocess of an application out of another.
One way to do it is to edit the login processing and create the cookie for the webmail when a user logs in. The catch is that when you upgrade ISPConfig it would blow up your upgrade.
