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A right-hand side blacklist (RHSBL) is a listing that contains the domain names of spammers, which mail servers can be programmed to reject. RHSBL functions the same way as a domain name system blacklist (DNSBL) with one important distinction: RHSBLs include domain names rather than IP addresses.
This type of blacklist gets its name due to the fact that the domain name in an email address appears to the right of the "@" sign.
Over the years, a number of DNSBLs have become vulnerable to denial-of-service (DoS) attacks. Those responsible for the attacks have not been uncovered, but many feel that spammers have been the ones to initiate them. In 2003, A DNSBL provider named Osirusoft was forced to shut down as a result of continuous DoS attacks. This outcome is likely to be exactly what those who perpetrate attacks against DNSBLs are hoping for.