[Mimedefang] rejecting invalid users

[Super-Dome]

Dear all 

i have following setup . 

Internet<------>Mail-Gateway<----->Internal-Exchange-mail-server
                

currently all invalid recipients emails go to mail gateway and then exchange
server bounce it  . i want to reject invalid recipients at mail gateway

i have found a doc to do this with Postfix using ldap to get user list from
domain controller etc

http://www-personal.umich.edu/~malth/gaptuning/postfix/

can i do this with mimedefang or sendmail ??

Regards

SD