A Domain Name System Blacklist (DNSBL) is a list that allows internet service providers (ISP) and other website administrators to block emails or traffic from specific systems. It's usually those that are known for sending spam and other malicious content. Normally, a blacklist contains domains, email addresses or IP addresses.
Having a domain blacklisted can prevent emails being sent. Instead, 'bounce back' messages are received from the server that has rejected the email.
