If you run a mail server (which I think everyone one should for a real decentralized communication network e-mail was originally designed for) and using postfix and use dnsbl.njabl.org to lookup blacklisted hosts when processing mail, then you need to remove this host as dnsbl.njabl.org has been sunset and will no longer resolve. This will affect your mail server as it will reject all mail received. Update your postfix configuration file /etc/postfix/main.cf and remove this RBL client entry e.g:
smtpd_client_restrictions = reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nl, reject_rbl_client dnsbl.njabl.org
Then restart your postfix server
/etc/init.d/postfix restart
Refer: https://en.wikipedia.org/wiki/Not_Just_Another_Bogus_List
Personal Computing - Decentralized 