Mail Abuse Prevention System

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The Mail Abuse Prevention System (MAPS) is an organization 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.
• Non-confirming Mailing List (NML), marketers who use opt-out strategy.

The acronym MAPS is spam spelled backwards.