An email blacklist (or "blocklist") is a collection of domains and/or IP addresses blocked from sending emails to email users because of spam activity. There are different kinds of blacklists including internal ones run by webmail providers (Gmail, Outlook, etc.), external ones run by security companies (Proofpoint, Barracuda, Cloudmark, etc.), and finally, those managed by non-profit organizations (Spamhaus, SURBL, etc.).