How about qmail? (Score 1)

I have actually rolled out an exchange site of that same scale but for my brother-in-law's small business I setup qmail, www.qmail.org, as my first Linux project. Its great and easy to manage, I have rebooted the box maybe once and that wasn't for any problems with qmail. There is a mailing list where you could ask about scalabilty. All Exchange sites of that considerable size do it with a multitude of servers. I think you can make several qmail servers talk to each other. I wish there was something in the open-source community that had all the features of Outlook(except Journal). That's what is really needed to switch the PHB's around.