Also there is skyrix.

http://www.skyrix.com/en/index.xhtml

Not cheap, but it is a full replacement of exchange server, and then
some.
There are other things out there. I just can't find them right now.
There are also many ways of putting several pieces together to make what
want.

A scheduler is here (also not free as in beer):
http://www.crosswind.com/oview.htm
and all you would need to do is configure it to work with say postfix.
And voila!

Brian
> -----Original Message-----
> From: Jeremy [mailto:enigmax@toocool.com]
> Sent: Sunday, March 24, 2002 2:53 PM
> To: Aaron
> Cc: kclug@kclug.org
> Subject: Re: Linux Replacement for Microsoft Exchange
> 
> 
> Bynari Insight Connector allows you to use any IMAP4 based 
> server with MS 
> Outlook (it's an extension to) that looks and acts *just* 
> like an Exchange 
> server, from scheduling calender / free busy etc.
> 
> http://www.bynari.net
> 
> you could use their server (costs money), cyrus, Caldera's 
> Volution, and SuSE 
> Email Server 3 (I think.)
> 
> -Jeremy
> 
> On Sunday 24 March 2002 10:39 am, you wrote:
> > Here's a question for the group...
> >
> > My boss is having fits with his exchange server.  I 
> suggested a Linux
> > alternative for mail, like sendmail or qmail.  He's all for 
> it and would
> > switch in a heartbeat, however, certain departments need 
> the collaborative
> > side to sync their appointments, contacts, etc....
> >
> > The question is.  Has anyone heard of a Linux product that 
> will replace all
> > the features of exchange Server while still allowing the 
> client side to
> > remain on exchange?
> >
> > Any help is appreciated.
> >
> > Aaron
> 
> 
> majordomo@kclug.org
>