A blacklist is a list used to identify IP addresses or domains that are known to send spam. Since 85% of messages sent everyday are spam, it protects users from receiving unnecessary spam emails. Companies can sometimes end up on blacklists unintentionally. They can end up on one of these due to - spam complaints, bad lists or addresses, or a surge in list size. Find out more in about email blacklists and how to prevent being on one, in this section.