If you want to run Postfix as a blackhole mail server (accepting connection for every user on every domain) then all you have to do is add these settings to /etc/postfix/main.cf :

relayhost = 
relay_transport = relay
relay_domains = static:ALL
smtpd_end_of_data_restrictions = check_client_access static:discard

Note: Postfix is not acting as an open relay in this configuration because it discards mails and is not relaying them to the final destination.