[plug] looking for a groupware e-mail system

[Jon L.Miller]

We are looking for a groupware e-mail system where the backend runs on a 
Linux  server and the desktop runs either outlook  or another pop3 client.  
The purpose is to allow the client the ability to share address books.  Group 
calendaring is not an issue at the moment.  Currently they are using Sendmail.

Any suggessions other than Exchange, Lotus Note or GroupWise? 

TIA