postfix log messages: RBL lookup error: Host or domain name not found

I'm finding quite a few of these types of messages in my postfix log: 17:40:55 smtpd: warning: 34.77.82.185.b.barracudacentral.org: RBL lookup error: Host or domain name not found. Name service error for name=34.77.82.185.b.barracudacentral.org type=A: Host not found, try again 17:41:05 smtpd: warning: 34.77.82.185.hostkarma.junkemailfilter.com: RBL lookup error: Host or domain name not found. Name service error for name=34.77.82.185.hostkarma.junkemailfilter.com type=A: Host not found, try again 18:15:02 smtpd: warning: ptmail1.patrontechnology.com.dbl.spamhaus.org: RBL lookup error: Host or domain name not found. Name service error for name=ptmail1.patrontechnology.com.dbl.spamhaus.org type=A: Host not found, try again 18:40:27 smtpd: warning: 177.141.213.134.zen.spamhaus.org: RBL lookup error: Host or domain name not found. Name service error for name=177.141.213.134.zen.spamhaus.org type=A: Host not found, try again I am trying to understand if there is something wrong with my configuration or if these messages are (as I have read non-authoritatively elsewhere) merely a slightly weird way of indicating that the sender is not black-listed by the given service. Certainly some (maybe all) of the emails which generate these messages are genuine and are indeed forwarded correctly and successfully by postfix. These are the relevant lines of my smtp_recipient_restrictions: reject_rbl_client zen.spamhaus.org reject_rbl_client b.barracudacentral.org reject_rbl_client hostkarma.junkemailfilter.com=127.0.0.2 reject_rhsbl_helo dbl.spamhaus.org reject_rhsbl_sender dbl.spamhaus.org reject_rhsbl_reverse_client dbl.spamhaus.org