A client has asked for an online mail collaboration facility to be setup on their webspace.

Essentially they want to give access to the mail to people in various locations with the option to collaborate between the employees etc.

Open Source would be preferred.....but any recommendations or experiences with anything in this genre would be helpful.

Zimbra Opensource?

I did take a look at Zimbra but it's overkill for what these people need. So I was looking for something a bit more lightweight and basic if possible.