What are blocklists and why do they exist?

Blocklists are databases of IP addresses and domains known for sending spam. MBPs and filters query blocklists during mail processing, using listing status as one signal in delivery decisions. Listings indicate past problematic behavior.

Blocklists exist because spam remains pervasive. They provide shared threat intelligence, enabling receivers to benefit from others' detection work. When one organization identifies a spam source, blocklist inclusion helps everyone block it.

For legitimate senders, blocklists represent risk. Inadvertent listing from compromised systems, list hygiene problems, or mistaken identification can severely impact delivery. Monitoring blocklist status and understanding delisting processes is essential deliverability hygiene.