Last time I checked evolution was the only email client supporting MS Exchange.
Novell offers a system with groupwise as server and evolution as client. Check it out.
Novell has also a great solution callef HULA:
http://www.hula-project.org/Hula_Server
Hula is a calendar and mail server whose goal is to be fun and easy to use, while scaling effortlessly from small groups to large organizations with thousands of members. Its opensource.