A spam blacklist is a list of Internet Protocol addresses that are marked as sending spam emails. Email providers use spam blacklists to help protect users from spam emails. Emails sent from IP addresses on the blacklist are usually blocked from ever appearing in a user's inbox, resulting in an email experience that is hopefully free of spam.
Many spam blacklist providers track IP addresses that send spam, though in different ways. Some of these services charge money for companies to access their blacklists, whereas others provide the information free of charge. Spam blacklist companies include Spamhaus, Barracuda, Spamcop and SURBL.
The technology used by spam blacklist providers is extremely advanced, and it often operates by monitoring how many people report emails as spam. If enough users report mail from a particular IP address as spam, the blacklist provider adds that IP address to its blacklist to help curb the amount of spam emails coming from that address. If an IP address is added to a blacklist, it can harm the reputation of the hosting provider in charge of that IP address. For that reason, most hosting companies pay close attention to spam blacklists and try to ensure users don't inadvertently get added to the blacklist.