Linux Replacement for Microsoft Exchange

[Aaron]

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