The Mail Abuse Prevention System (MAPS) is an organisation that provides anti-spam support by maintaining a DNSBL. They provide five black lists, categorising why an address or an IP block is listed: Real-time Blackhole List (RBL), the one for which MAPS is probably best known. Dialup Users List (DUL), blocks of addresses that include many SOHO users. Relay Spam Stopper (RSS), spam relays, e.g. hijacked servers. Open Proxy Servers (OPS), naively open SMTP servers.