Postfix can directly query Active directory and the setup is much simpler.  In addition, your postfix setup is now always up-to-date on active users.  In main.cf add:

relay_recipient_maps = ldap:/etc/postfix/ldap.cf

in ldap.cf:

server_host = ldap://dc1.example.com:389
search_base = cn=Users,dc=example,dc=com
timeout = 5
bind = yes
version = 3
bind_dn = ADuser@example.com
bind_pw = ADusersPW
query_filter = (proxyAddresses=smtp:%s)
result_attribute = extensionAttribute1
domain = hash:/etc/postfix/transport

example.com smtp:exchange.example.com:25

Lastly in the Active Domain user management you need to put "OK" (no quotes) in the extensionAtribute1 for each user you wish to allow to receive external email.  This is required because Postfix looks for OK in the lookup response, anything else is a negative response.

Enjoy,

Woody (woody at linif dot org)