The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer's use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.
PBL IP address ranges are added and maintained by each network participating in the PBL project, working in conjunction with the Spamhaus PBL team, to help apply their outbound email policies.
Additional IP address ranges are added and maintained by the Spamhaus PBL Team, particularly for networks which are not participating themselves (either because the ISP/block owner does not know about, is proving difficult to contact, or because of language difficulties), and where spam received from those ranges, rDNS and server patterns are consistent with end-user IP space which typically contain high concentrations of "botnet zombies", a major source of spam. Once aware of them, the ISP/block owner can take over such records at any time to manage them further.
The PBL lists both dynamic and static IPs, any IP which by policy (whether the block owner's or -interim in its absence- Spamhaus' policy) should not be sending email directly to the MX servers of third parties.
IP Address Self-Service Removal Mechanism
A feature of the PBL is the elimination of 'false positives' with a server-identifying and automatic removal mechanism for single IP addresses. This allows end users with static IP addresses within a larger dynamic pool, and legitimate mail server operators, to assert that in their opinion their IP addresses are a trustworthy source of email and to automatically remove (suppress) their IP addresses from the PBL database. Safeguards are built in to prevent abuse of this facility by spammers (and particularly by automated bots).