`non_fqdn_reject_code`

This parameter specifies the SMTP server response code when a mail violates the `reject_non_fqdn_hostname` or `reject_non_fqdn_sender` or `reject_non_fqdn_recipient` rule that are:
Access denied: Invalid domain for client HELO hostname, sender or recipient email address (not FQDN)
Note that the postfix default value used for `non_fqdn_reject_code` is code 504, and this code in 'standard RFC 822' should means 'Command parameter not implemented'. So with Postfix, a `non_fqdn_reject_code` rule violation does not reflect exactly the RFC822 explanation of reported code.

To avoid confusion, you can change this to 578 for example, otherwise, do not change this unless you have a complete understanding of RFC 822.