Outlook clients and Linux Debian

[Hervé Piedvache <bill.footcow@gmail.com>]

Hi,

I have at the office a part of Windows users using Outlook ... others are 
under Gnu/Linux Debian ... all the services (mail, files server, printer 
servers are under linux) ...
My Outlook users would like to share their calendars ... in fact when one make 
an appointment in his calendar with another guy he wants to know if the other 
guy have or not an appointment at the same time ...
For this the classical solution should be to add an exchange server ... but 
you might understand that I don't want to do this ...

What kind of solution could I find to this stuff under a Debian Linux 
service ?

Thanks per advance for your answers ...
-- 
Bill