Email users who find their messages blocked from mail servers that use DNSBLs often object vociferously, sometimes to the extent of attacking the existence of the lists themselves.

Indeed, there is some actual controversy on the following three:

  • Lists of dynamic and dial-up IP addresses. The administrators of some mail servers choose not to accept messages from dynamic addresses, since they are often home computers exploited by spam viruses. This can bring serious inconvenience and frustration to users who wish to run their own mail servers on residential ISP connections or local MTAs. As a work-around, users who wish to run their own mail server are expected to relay via their ISP’s mail server using the smart host functionality built-in to most mail servers. Furthermore, in some cases it may be quite tricky to get an erroneously listed IP address removed. For example, to request removal from the DUL provided by dynablock.njabl.org, you are supposed to send an email to removals at mail.njabl.org but that address is in turn protected by the same DUL you are asking to be removed from.
  • Lists that include spam-supporting operations. A spam-supporting operation is generally a site that may not directly send spam, but provides commercial services to spammers, such as hosting of web sites that are advertised in spam messages (spamvertized web sites). Refusal to accept mail from spam-supporting operations is intended as a boycott and aims to encourage such sites to cease doing business with spammers. Unfortunately, this is done at the expense of inconveniencing non-spammers.
  • Predictive (“early warning”) lists. Such lists contain addresses belonging to spam-supporting operations, under the hypothesis that such addresses are more likely to send spam in the future.

Many people have voiced vigorous objections to specific DNSBLs.

Some object to the principle that mail-receiving sites should be able to reject undesired mail systematically. One who does is John Gilmore, who deliberately operated an open mail relay. Gilmore accuses DNSBL operators of violating antitrust law:

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.

A number of parties, such as the Electronic Frontier Foundation (EFF) 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.

There are even cases where spammers have pursued lawsuits against DNSBL operators on similar grounds. In 2003, a newly-formed corporation calling itself “EmarketersAmerica” filed suit against a number of DNSBL operators in a Federal District Court in Florida. Operated by one Mark Edward Felstein, Esq., the company claimed to be a trade organization of “email marketers” and that DNSBL operators Spamhaus and SPEWS were engaged in restraint of trade. The suit was eventually dismissed for lack of standing.