I am using DNS-based Blacklist (spamhaus.org and spamcop.net) to filter out spam emails. The blacklist systems are based on IP addresses which somebody might call it Left Hand Side Blacklist (LHSBL). There is another kind of DNS-based blacklist system that check domain names instead of IP address. The name Right Hand Side Blacklist (RHBL) is therefore established as we look at a DNS record, the right hand side always denotes a domain while the left hand side tells the IP address associated.
Suppose an email has the domain name "example.com" in the "MAIL FROM:" field and the server dsn.rfc-ignorant.org is where we want to lookup. We will look at example.tld.dsn.rfc-ignorant.org, and if the IP address 127.0.0.2 is returned, the domain name "example.com" is blacklisted.
Ha... I have gained some new knowledge in the area of DNSBL.
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