SMTP server response on RBL domains violation
maps_rbl_reject_code

This parameter specifies the SMTP server response code when a mail violates the maps_rbl_domains rule that is:
Access denied: SMTP client listed in RBL

Note that the postfix default value used for maps_rbl_reject_code is code 554, and this code in 'standard RFC 822' should means 'Requested mail action rejected: access denied'. So with Postfix, a maps_rbl_domains rule violation does not reflect exactly the RFC822 explanation of reported code.

To avoid confusion, you can change this to 571 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.