Linux Mail Server Setup and Howto Guide

OHAI!

Working on a project for our Computer Systems Technology course – establishing a CentOS 6 server to function as a mail server for our classroom (class uses Microsoft Outlook as eMail clients, one system is running Ubuntu 11.10 with the native mail handler).  So far, have postfix and dovecot up and running, and WebMin for remote server administration.

At this point is where what knowledge I've gleaned from this and other sites fails.

I'd like to configure the server to work as a collabroation server; that is, the server will host a unifed caledar for the class, as well as function as a general intranet mail server for the classroom (as a training environment).  I've heard Zimbra is the way to go if seeking an open source solution; however, my skills with any form of Linux are underdeveloped at best description, and I've been running into dead ends.  Could one of you point me toward some plain English 'how tos' as to setting up a local intranet mail and collaboration server?