Ninguém menos do que estimado Autor do Postfix Wietse Venema nos dá a resposta :
Perhaps with a transport table, taking advantage of its special syntax:
/etc/postfix/main.cf transport_maps = hash:/etc/postfix/transport /etc/postfix/transport: example.org : * error:we do not deliver this domain
The above will reject SMTP mail, and bounce local submission, for domains other than example.org.