There are a huge number of blacklists on the Internet. In short they are all attempts to block certain types of users and activity from your computer, e-mail address or website by providing a list of suspected problem IP addresses. Various official and unofficial bodies maintain these lists and the quality of their data collection and the exact nature of their content varies considerably. During the days of the file sharing boom many users downloaded blacklists containing IP addresses of suspected anti-piracy groups to block them from connecting to their PC to check their file shares.
As an Internet marketer your biggest concern will be delivering a high number of your e-mail messages to clients. Having your e-mail server blacklisted as one delivering SPAM (unsolicited e-mail) would hugely impact your delivery rates as most big e-mail providers check various e-mail blacklists before delivering mail to their customers. As a result it is important that professional Internet marketers follow a diligent opt-in e-mail strategy and send their subscribers useful relevant information and advertisements to avoid excessive SPAM reports and any danger of blacklisting.