Which Mail Server is best?

We are currently using Window’s Workgroup Mail, as our mail server on our NT4 network. Clients are running ’95 and ’98. I have 125 mail users now. I know it’s far past time to get a “real” mail server running. I am interested in the pros and consabout the choices I have to pick from, for a mail server progam. I know Exchange and Lotus Notes are popular. What would someone tell me, to sway me one way or another? And, are there other mail server programs I should consider?
Thanks!