`relay_domains_reject_code`

This parameter specifies the SMTP server response code when a mail violates the `relay_domains_reject` rule that is:
Access denied: Relay not authorized or not local host not a gateway
Note that the postfix default value used for `relay_domains_reject_code` is code 554, and this code in 'standard RFC 822' should means 'Requested mail action rejected: access denied'. So with Postfix, a `relay_domains_reject` rule violation does not reflect exactly the RFC822 explanation of reported code.

To avoid confusion, you can change this to 572 for example (another reason is that code 554 is also the postfix default value for other kind of errors), otherwise, do not change this unless you have a complete understanding of RFC 822.