Regarding spam, blacklists are lists of known spammers, their IP addresses, and/or their ISP (Internet Service Provider). Using this information, spam filters can block all messages coming from known spammers and/or their ISPs. ISPs that fail to discipline spammers may find all email from their legitimate customers blocked by large numbers of recipients. This tactic forces the ISP to take action against spammers using their systems because legitimate users do not want to be inconvenienced by having their email blocked. The opposite of a whitelist.