Consider Openwave or Sun Enterprise Mail Platform (Score 1)

I'd check out the Sun Mail platform or the Openwave platform. They are pay for, but scale very large! Sun also licenses per employee instead of per mailbox, which is a plus for email providers. The Sun Platform has a plugin for Outlook that allows it to totall mimic outlook back end things like calander and such. http://www.openwave.com/ http://www.sun.com/software/javaenterprisesystem/i ndex.xml Just my opinion, For what it's worth.