Some MS Exchange alternatives:

• Zimbra [zimbra.com]
• Scalix [scalix.com]
• eGroupware [egroupware.org]
• PHP Groupware [phpgroupware.org]
• Communigate Pro [communigate.com]
• Citadel [citadel.org]
• Kolab [kolab.org]
• Open Groupware [opengroupware.org]
• Open xChange [open-xchange.com]
• Zarafa [zarafa.com]