A Dial-up/Dynamic User List (DUL)[1] is a type of DNSBL which contains the IP addresses an ISP assigns to its customer on a temporary basis, often using DHCP or similar protocols.
Their primary function is to assist an ISP in enforcement of its Acceptable Use Policy, many of which prohibit customers from setting up an email server[citation needed].
Receivers adopt such policies because computers at dynamically assigned IP addresses so often are a source of spam.
The DUL subsequently was absorbed by Mail Abuse Prevention System (MAPS) in 1999.
When MAPS was no longer a free service, other DNSBLs such as Dynablock, Not Just Another Bogus List (NJABL), and Spam and Open Relay Blocking System (SORBS) began providing lists of dynamically assigned IP addresses.