A DNS blacklist, also known as a domain name system blacklist, is a software technique that aids or assists mail server providers in blocking IP addresses that transmit spam.
Anti-spam organizations may ban a domain if it sends too many unsolicited/phishing emails. When a domain gets banned, it is more likely to be viewed as a spammer. There are programs available, such as mail-tester, that can assist you in determining whether or not your domain is blacklisted.