The Abusix blacklist uses its proprietary formula to identify spam sending mail servers. It uses several different zones to classify the type of problems it has seen with an IP address.
A listing by Abusix Blacklist indicates the IP addresses of any hosts that have recently sent spam to Abusix trap domains. Their traps have never been used for mail and never originate mail, and Abusix never uses typo domains (e.g., domain names similar to large services are blacklist traps).
Therefore, any host sending mail to Abusix traps are either infected, compromised, spam from purchased lists, or spam from services that do not perform confirmed opt-in (e.g., validating the email address of new sign-ups before allowing access or adding the address to CRM systems, etc.).
To avoid listing large mail providers such as Google, Microsoft Office 365, or any other multi-tenant mail services that might have compromised accounts or fraudulent sing-ups, Abusix has extensive whitelists that prevent these hosts from being listed.