Almost everyone now has a mail system in place, even if they don't have an intranet. As the selection of an e-mail technology depends on many factors, including personal preference, we have no desire to impose our choices on our clients. There are many good commercial products which provide online calendars, meeting bookings, and other such features. If these products work on the web, most probably they can be integrated with Illumination. As Illumination runs using the very common combination of Apache and Linux, it can be considered reasonably secure.
Illumination is, however, designed to provide intranet services only.
We would therefore recommend configuring it so that it is not accessible from the outside. Alternatively, you may wish to consider installing a firewall to protect both Illumination and the rest of your network. In short, while we are fairly confident of the security in practice, we make no guarantees. It is your responsiblity to protect the Illumination machine to the same degree that you protect the rest of your network. Many of these provide collections of applications, accessible via a web browser.