So my question is which directive to put rbl settings in? "smtpd_recipient_restrictions" or "smtpd_client_restrictions"?
Dealers choice. Both work, just depends when you want them rejected. If you wait for recipient you can see in the logs who the mail was going to. If you kick them during the client process they never get the chance to provide the who. Look into smtpd_delay_reject for more understanding on this.