Need Help on Mail Server Software

Hi Frieds,
Well My company is going to have a inhouse Server soon..The company is planing to host all its mail domains on a single machine..currently we have 8 mail domains.Could you please suggest whether i can go with Microsoft Exchange server or any other good servers are available in the market..If any of the Guru’s can explain me the pro’s n cons it’d be of a grt help to me…
Thx in Advance