I'm looking for a solution like an exchange server for email, calendar, contacts, etc. I would prefer to use a open source solution. Any suggestions?
The client PCs are using Mac/Unix and Windows, so the server must be accessible from all platforms. I prefer to used a non-Web-based solution, but i'm open to web-based suggestions if they provide all the needed functions (email, calendar, contacts).
It's not open source, but Kerio MailServer is a good, cross platform alternative to Exchange, particularly for smaller businesses.
You can install a set of services like ldap, caldav, postfix and glue all them with horde.
Look into Funambol. It is open source. Not sure it does everything but it may be a good start.