[1] Most mail server software can be configured to check such lists, typically rejecting or flagging messages from such sites.
Since the creation of the first DNSBL in 1998, the operation and policies of these lists have frequently been controversial,[3][4] both in Internet advocacy circles and occasionally in lawsuits.
Many email systems operators and users[5] consider DNSBLs a valuable tool to share information about sources of spam, but others including some prominent Internet activists have objected to them as a form of censorship.
[6][7][8][9] In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down.
[10] The first DNSBL was the Real-time Blackhole List (RBL), created in 1997, at first as a Border Gateway Protocol (BGP) feed by Paul Vixie, and then as a DNSBL by Eric Ziegast as part of Vixie's Mail Abuse Prevention System (MAPS); Dave Rand at Abovenet was its first subscriber.
Before an address would be listed on the RBL, volunteers and MAPS staff would attempt repeatedly to contact the persons responsible for it and get its problems corrected.
Such effort was considered very important before black-holing all network traffic, but it also meant that spammers and spam supporting ISPs could delay being put on the RBL for long periods while such discussions went on.
However, many observers believe the attacks are perpetrated by spammers in order to interfere with the DNSBLs' operation or hound them into shutting down.
In August 2003, the firm Osirusoft, an operator of several DNSBLs including one based on the SPEWS data set, shut down its lists after suffering weeks of near-continuous attack.
However this is typically inefficient for zones containing large numbers of addresses, particularly DNSBLs which list entire Classless Inter-Domain Routing netblocks.
For Joe Blow to refuse emails is legal (though it's bad policy, akin to "shooting the messenger").
But if Joe and ten million friends all gang up to make a blacklist, they are exercising illegal monopoly power.
[24]A number of parties, such as the Electronic Frontier Foundation and Peacefire, have raised concerns about some use of DNSBLs by ISPs.
One joint statement issued by a group including EFF and Peacefire addressed "stealth blocking", in which ISPs use DNSBLs or other spam-blocking techniques without informing their clients.