If it doesn't absolutely have to be Open Source / Free Software, take 
a look at http://www.bynari.com

Their Insight product is reasonable, reliable, runs on Linux / UNIX and 
interoperates with Outlook and a whole bunch of other things. Does 
everything Exchange does but costs lots less and runs on Linux.

I have no affiliation to Bynari, just like their products...



-=[ Steve ]=-



Mike Nielsen wrote:

> Hi there.  I make my living bouncing around the Twin Cities setting up
> companies (5-100) users with networks and servers and the like.
> 
> As such I have begrudgingly only offered up support for Microsoft products, 
> with the exceptions of SNAP servers, firewalls, etc. etc.
> 
> For my smaller clients who are on a tight budget I would like to be able
> to offer them a messaging server that has much of the same functionality as 
> exchange.
> 
> Calendar Sharing, contact management, email, task lists...   I haven't found 
> a lot of products out there that meet that criteria.  
>  
> I'll continue to peek around Freshmeat but any place you all can point me
> would be greatly appreciated.