postfix authentication against Active Directory
we are in the process of converting our company mail server MS Exchange with open source (postfix). Our company use Microsoft Active Directory Services and did not wish to introduce another form of user directory to store identities and passwords. It mean Postfix have to authenticate users against AD, Using Active Directory to obtain authentication means that users of the system don't have to remember a separate username and password just for the email system.
What I'm looking for a HOWTO for postfix + imap + AD, I google but didn't find anything interesting.
I will greatly appreciate a link/url/reference, suggestion in this regard.