DeskNow - Mail and collaboration server
DeskNow is a fully featured and easy to use mail and collaboration server that can handle all the communication and collaboration needs of your company.

DeskNow integrates in a single, easy to administer product: DeskNow can be installed in minutes, on Windows, Linux, FreeBSD, Solaris, Mac OS X, etc.
Advanced calendaring with meeting planning, free/busy search, shared calendars, task lists, resource booking, iCal publishing (Mozilla Sunbird, Apple iCal)
Outlook connector to integrate personal and group calendars, tasks, contacts SyncML synchronization with Palm, PocketPC, BlackBerry and most mobile phones, secure Jabber instant messaging with integrated web client automatic authentication and account import with Active Directory and LDAP servers
- Fully featured mail server
- Advanced antispam technologies, antivirus integration and content filtering
- Dynamic webmail interface
- Digital email encryption and signing
- Document management
Free version
After the trial period is expired, you can continue to use DeskNow in Lite mode, forever. This is perfect for home use or companies with zero budget. Or you keep all the Professional features by purchasing a license.
( i tried it a few years back and it is a professional, well engineered piece of software or webware, it can be used as a simple intranet application for small business - San Gear)