good mail server to replace exchange

Hi,

I am trying to get the company I work for to migrate away from exchange email servers. The mail server software seems buggy, and doesn't always perform nicely in our mixed computer enviroment (macs, pcs, linux).

SO, I'm going out on a limb to say that I want to switch them over to an open source. So far, there has been some resistance. I need to know a great mail server to use, and possibly reasons why it is better. My sysadmin can list dozens of compnies that use Exchange and always says "I don't think it's wise to trust our companies email with open source"