Email Server Software? Which one? Why?

I am the head of IT at my company and I am in the process of
planning a corporate email solution. I have to choose between
three different options… Exchange (which is low on the list), Lotus
Notes, Squirrelmail. I really would like to use Lotus Notes but
because of cost I am looking strongly at Squirrelmail. I have about
40 people that will be using this server everyday. I would like to
hear opinions about which software is great to setup, maintain, and
forget for a while. Also ease of use for the client.