You can build software packages for ISPConfig that integrate into ISPConfig, and that can be installed using the Update Manager. For instructions, have a look here:
http://www.howtoforge.com/wiki/index...8.pkg_files%29
or unzip the webmail package (the .pkg is a simple zip file) and have a look at its contents.
Of course, you could also simply create a web site in ISPConfig and install your webmail in that web site and give your customers the URL.
