I agree with Abecx that MS makes a bad mail platform. Mail servers, especially busy ones, shouldnt have any downtime, and every little update for M$ requires a reboot. Linux works really well, you just have to restart the mail service which takes about 3 seconds.
I use Exim as well but with pop3 and a MySQL backend since I host alot of different domains and it makes it really easy to maintain. My email server hasnt been rebooted in like 9 months.