Zarafa is an opensource drop-in replacement for Microsoft Exchange.
Zarafa offers support for Microsoft Outlook, generic email clients via IMAP and Caldav, mobile devices and has AJAX based WebAccess.
The Yubikey integration offers a two-factor authentication for the Zarafa WebAccess.
The integration project is available on
http://community.zarafa.com/pg/plugins/project/5102/developer/milo/yubikey-zarafa-authentication-integrationMore information about Zarafa Collaboration Platform, can be found on
http://www.zarafa.com.