`reject_code`

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

To avoid confusion, you can change this to 575 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.