Cannot receive emails: Client host rejected: Service unavailable

Symptoms

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

   CONFIG_TEXT: 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 :

   CONFIG_TEXT: postfix/smtpd[5105]: warning: 192.0.2.2.dnsbl.example.com: RBL lookup error: Host or domain name not found. Name service error for name=192.0.2.2.dnsbl.example.com 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.com /etc/postfix/main.cf smtpd_recipient_restrictions = ...permit_dnsbl_client dnsbl.example.com=192.0.2.2,

Cause

Plesk is configured to use non-existent DNSBL service.

Resolution

Remove non-existent DNSBL in Plesk > Tools & Settings > Mail Settings:

Also, it can be done using command line:

1. Log in to Plesk using SSH.

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

   CONFIG_TEXT: # permit_dnsbl_client list.dnsbl.example.com=192.0.2.2

3. Restart Postfix:

   # service postfix restart