In the past I’ve used The Hula Projet as a mail server, but one thing has always lacked in terms of real production readiness in my opinion – Outlook Integration.
Many Open Source mailservers (including hula) support a protocol called Caldav. You can connect to Caldav servers via several open source calendar like Sunbird.
The Open Soiurce Outlook Connector Project is a plugin for Outlook that translates the Caldav stream into a format that Outlook can understand.
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Jon Howe