The IT Law Wiki


Content filter[]

Black lists are

lists of URLs or Internet Protocol ("IP") addresses that a filtering company has determined lead to content that contains the type of materials its filter is designed to block.[1]


A black list is

[a] list of discrete entities, such as hosts or applications, that have been previously determined to be associated with malicious activity.[2]


A black list is "[a] list of email senders who have previously sent spam to a user."[3]

Spam filter[]

A blacklist (also called a DNS Blacklist or DNSBL) is a list of IP addresses from which spam originates that is used to block email from those addresses. The list is made available to ISPs that operate mail servers so that they can block emails sent to or from those IP addresses. Mail servers may be configured to refuse email coming from IP addresses, IP ranges or whole networks listed on a specific blacklist.

Overview (Spam filter)[]

Most of the lists are free and run by volunteers, though their operations may be funded through external sources. Each blacklist has its own criteria for including an IP address in the list and its own procedure for getting an address off the list. Spamhaus, an international nonprofit organisation funded through sponsors and donations, maintains several well-known blacklists — though they prefer the term block lists — which they claim are used to protect over 600 million user inboxes. One of their lists contains the addresses of “spam-sources, including spammers, spam gangs, spam operations and spam support services”; another list focuses on botnets which run open proxies.

The most common form of black list is a Domain Name System black list (DNSBL), a list of IP addresses distributed via the Internet's DNS. Popular DNSBLs include the Spamhaus Black List (SBL), the Composite Blocking List (CBL) and the original DNSBL, called the Mail Abuse Prevention System (MAPS) Reverse Black List (RBL).

Blacklisting, while potentially powerful, has drawn its own criticisms — regarding, among other things, vigilantism of blacklist operators, listing false positives, the collateral damage that may come with blacklisting certain IP addresses or IP ranges, and the financial motives of some list operators. Blacklists have faced legal challenges from spammers, who on occasion have been successful in obtaining court verdicts against being blacklisted.

Despite these issues, most ISPs and e-mail servers use blacklists.


  1. American Civil Liberties Union v. Gonzales, 478 F.Supp.2d 775, 790. 775 (E.D. Pa. 2007), aff'd, 534 F.3d 181 (3d Cir. 2008), cert. denied sub nom. Mukasey v. American Civil Liberties Union, __ U.S. __, 129 S.Ct. 1032 (2009).
  2. NIST Special Publication 800-94.
  3. NIST Special Publication 800-114.