The Mail Abuse Prevention System (MAPS) is an organization that provides anti-spam support by maintaining a DNSBL.
Dave Rand and Paul Vixie, well known Internet software engineers, started keeping a list of IP addresses which had sent out spam or engaged in other objectionable behavior.
Doing this would bring various advantages and disadvantages; Acknowledged ISPs can, in general, afford to monitor their systems more thoroughly in order to avoid viruses, hijackers and similar threats.
Furthermore, it paves the way for effectively exploiting policies like SPF, which rely upon end-user SMTP authentication in order to block email address abuse.
MAPS fails to disambiguate the concepts of acknowledged ISP versus end-users of IP addresses with a formal definition.
When coupled with the ability to easily whitelist IPs by local Internet registry/region to correct obvious shortcomings, using the DUL to block mail may result in an obscure policy that jeopardizes the global reliability of email delivery.
It generates an amount of false positives much higher than MAPS claims to be aware of, blocking many legitimate websites and end users, and yet catching only an estimated 2% of spam.