Hello, I don't know if you want to go this far as it requires slight customization, but my setup is postfix+postfixadmin+dovecot with owncloud providing storage (webdav), calendar (caldav), and contacts (carddav) as well as a nice web interface if you prefer to use that. Roundcube provides webmail as an option too. Z-push provides a completely seamless exchange-compatible front-end to calendar, contacts, and email. With the right DNS records and web-server aliases, z-push even passes microsoft's auto-discovery tool checks. I've got them all integrated to use the same user/pass from the postfixadmin database.
You could even tell people it's exchange and they wouldn't know from the front-end, and best of all it's all FOSS. I'm not sure if a big corporation would go for it, but it is an option. If you want more details or example configs or anything, let me know. Good luck! Travis
