This solution is a bit limited as you rewrite the default_host by looking which domain it is called from. But this isn't possible if you install it centrally via e.g. "webmail.serviceprovider.com" not only because of the www-part, but also because a central domain for all customers wouldn't hold any additional information. With my plugin approach, RC selects the server from the ISPConfig database so it will always match the right one, even if you move a user from one server to another. Only thing left is the login mask. I have an idea about that, but that will have to wait until tomorrow.