heavy traffic portal site

Hi there We have been contacted about creating a portal site which will have some heavy usage. They are talking about having 100,000 subscribed users to the system which will have the following: 1. Web based email 2. Calender (for the persons own use, not shared) 3. File store (and sharing) and image store (and sharing) and a few other things. The above are the areas that we think would create the most "load" on the system. Firstly, does anyone have an Open Source application that they would recommend for the above and are there any examples of other people using this in the real world? If there is no application that would handle all of the above, what would people suggest? Thanks Ade