`invalid_hostname_reject_code`

This parameter specifies the SMTP server response code when a client violates the `reject_invalid_hostname` anti-UCE restriction rule that is:
Bad syntax for client HELO hostname (Not RFC compliant)
Note that the postfix default value used for `invalid_hostname_reject_code` is code 501, and this code in 'standard RFC 822' should means 'Syntax error in parameters or arguments'. So with Postfix, a 501 status code does not reflect exactly the RFC822 explanation.

To avoid confusion, you can change this to 574 for example (another reason is that code 501 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.