What are blacklists?

Databases used by antispam security mechanisms to filter incoming messages. They include all those email addresses, IPs or domains whose email communications are rejected because they are not considered legitimate or trustworthy. Email providers perform this classification with the collaboration of users, who can block incoming messages and censor those issuing accounts that have bypassed the filters or did not have their approval. Repeated spam complaints from users can lead to a legitimate email address being blacklisted.