The DUL is the Dial-up User List. It is a list of ip address pools that contain spammers who use dial-up accounts or other means of dynamic IP addressing to mask their activities. The list contains entire pools of IP addresses. If a single person at your ISP is on the list, then the entire ISP's dial-up address space is on the list.
Obviously, this can cause problems for innocent users. A person who filters out the DUL also filters out many people who are not actually spammers, and will be unable to recieve mail from them.
The DUL is also occasionally used as a hosts.deny list.