Quick reference for using postfix as a “shield” server to Exchange to check valid mailboxes against ActiveDirectory (SBS edition):
create a LDAP configuration file:
server_host = ad-server.your.domain
search_base = ou=MyBusiness,dc=your,dc=domain
query_filter = (&(objectClass=*) (proxyAddresses=smtp:%s))
result_attribute = sAMAccountName
bind=yes
bind_dn = dn of user account
bind_pw = pw of user