Postfix snapshot 2.12-20141126 changes the error messages for domains with a NULL MX record from "invalid DNS reply" to "domain does not receive mail".
This also introduces a new SMTP server configuration parameter
nullmx_reject_code (default: 556) and updates the documentation
for reject_unknown_sender/recipient_domain.
I will upload a minimal patch for older Postfix versions later.
Wietse
