Spam Prevention Early Warning System

The Spam Prevention Early Warning System (SPEWS) was an anonymous, volunteer-operated service that maintained a list of IP address ranges belonging to internet service providers (ISPs) that hosted spammers, were considered spam-friendly, or hosted websites advertised in spam. Internet sites could query the list and use it as an additional source of information when deciding whether to block or filter incoming email.

Source: Wikipedia — Spam Prevention Early Warning System (CC BY-SA 4.0)