How to link a Linux Mail Server to a Windows Domain for email accounts?

Hi Linux experts,


My office has a linux (CentOS) email server running Sendmail & Dovecot IMAP.  It was configured by the IT Manager so that when a user is created on the Windows 2008 domain, the same user email account will also be available in the Linux Server.

Example:
- domain\john.doe user is created on AD
- john.doe@linux-email.com is automatically available in the Linux Email server.  
- The login will be same as AD's (john.doe), and password is also the same as the AD's

I want to learn the configuration behind it.  However I don't know where to start to look for information.  I was wondering if it uses the OpenLDAP, but as I followed the directions from OpenLDAP in CentOS, I don't see any configs done in those config file.  So, I think it might not be using OpenLDAP.

http://www.centos.org/docs/5/html/Deployment_Guide-en-US/s1-ldap-quickstart.html

Can anyone suggest what could be the mechanism for this auto linkage?  Sorry, I am a novice in Linux, hopefully I am expressing myself OK.  Thank you.