SMTP server response on fqdn violation
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.