Linux Mail Server Setup and Howto Guide

Alright, so here is a quick backstory to what's happening here:

I work for a school district in Alberta, and we're in the process of migrating our email from Novel Netmail to Microsoft Exchange 2010, but it turns out our licensing agreement doesn't cover student mailboxes, so we'd be looking at more cash than the current budget allows for, so now we're looking to find another solution for the student population.

While I've been able to get a Linux mail server up and running no problem, what I haven't been able to find an answer for is can an Exchange Server and a Dovecot/Postfix Server share a common Active Directory LDAP to manage the users.  We're really trying to avoid having to recreate the users (Students) in both Active Directory and then in the Linux system for their email.

Thanks for reading and thanks in advance for any suggestions/help.

Robin