Yes it is a Server based solution with desktop client software (think Exchange/Outlook) and only uses IMAP.
You can virtualize a server though and use Samba on Linux. If you you are feeling really keen you could set up a multiple server box with VMWare ESXi for free!
Looks interesting though but how long will good forum support last - they are selling support at large corporate prices, and if you want it without advertising support it will cost $50 per year per desktop (or $36000 for an unlimited perpetual license).