Symptoms

1. Cannot receive emails. The following error may be found in /var/log/maillog :

   postfix/smtpd[5105]: NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 450 4.7.1 <203.0.113.2>: Client host rejected: Service unavailable; from=<john.doe@example1.com> to=<jane.doe@example2.com> proto=ESMTP helo=<[203.0.113.2]>
   

2. Warnings in /var/log/maillog :

   postfix/smtpd[5105]: warning: 192.0.2.2.dnsbl.example.tld: RBL lookup error: Host or domain name not found. Name service error for name=192.0.2.2.dnsbl.example.tld type=A: Host not found, try again
   

3. The Postfix configuration parameter smtpd_recipient_restrictions is configured to use DNSBL service dnsbl.example.tld in /etc/postfix/main.cf file:

   # grep dnsbl.example.tld /etc/postfix/main.cf
   smtpd_recipient_restrictions = ...permit_dnsbl_client dnsbl.example.tld=192.0.2.2, 
   

Cause

The Postfix is configured to use non-existent DNSBL service.

Resolution

1. Remove non-existent DNSBL service dnsbl.example.tld from Postfix configuration in /etc/postfix/main.cf :

   permit_dnsbl_client list.dnsbl.example.tld=192.0.2.2, permit_dnsbl_client list.dnsbl.tld=192.0.2.2,<\pre>

2. Restart Postfix:

   # service postfix restart