Overview
SORBS DUL (Dynamic User List) was a policy-based sub-list within the SORBS system that cataloged IP address ranges assigned to residential broadband, mobile networks, and other dynamic allocations. The policy rationale is that legitimate mail servers use static IPs and properly configured PTR records β IPs from dynamic consumer pools should not be connecting directly to mail servers.
SORBS DUL was widely used as a way to block spam from compromised consumer machines without needing observed spam evidence. SORBS shut down all services in 2024.
How It Works
SORBS DUL was populated using IP allocation data from ISPs, WHOIS records, and reverse DNS patterns indicative of dynamic assignment (e.g., PTR records like pool-xxx-xxx.dsl.isp.net). Unlike spam detection lists, DUL listings were based on IP category rather than observed abuse, meaning legitimate servers on dynamic IPs could be incorrectly listed.
How to Get Delisted
SORBS is no longer operational and DUL cannot be queried or modified. For mail servers on dynamic IP ranges, the best practice remains obtaining a static IP, a matching PTR record, and listing on ISP whitelist programs. Spamhaus PBL covers similar functionality with active maintenance.