If you are using Exchange 2000 or Exchange 2003, it uses the Active Directory for its address book.
If you are using Exchange 5.5 (an older and probably no longer supported product), it provides it's own directory service that is also accessibile via LDAP. You may be able to find information on it's LDAP implementation at http://www.microsoft.com/technet/prodtechnol/exchange/55/reskit/exc00.mspx