At last, an online demonstration of Modoboa is available for testing! It is gracefully hosted here, many thanks to Patrick Koetter.
A link to the demo has been added on this page, into the right column.
The content is resetted every two hours, you'll be able to test almost everything. If you find any issue and if you want to discuss about functionalities, do not hesitate! You can post to the mailing list or join the IRC channel #modoboa on freenode.