Josep just let me know about this company PostPath that I had not heard about before. It turns out that PostPath is attempting to replace Microsoft Exchange with their low cost Linux Mail server. I was really shocked by the claimed feature set but I will reserve judgment until I see it in action. Supposedly the server even supports BES & Active-sync connectivity!
If you have been trying to move away from Exchange this may be worth looking at in more depth. Some of this is just too hard to believe. They will be at WES if somebody wants to check them out.
For more info and a 12 User free trial check out their website www.postpath.com