The OpenChange MAPI library aims to provide interoperability with an 
Open Source implementation of Microsoft Exchange protocols under UNIX/Linux.
The current implementation offers a client-side library which can be used in
existing messaging clients and offer native compatibility with Exchange Servers
up to 2007.

MAPI stands for Messaging Application Programming Interface and is used in the
Microsoft Exchange Server groupware server. This solution works with Outlook
and provides collaborative features such as a messaging server, shared 
calendars, contact databases, public folders and tasks.

WWW: http://www.openchange.org