What are content-based blocklists (SURBL, URIBL)?

SURBL (Spam URI Realtime Blocklists) and URIBL are specialized blocklists that track domains appearing in spam message bodies. Unlike IP-based blocklists that flag sending servers, these lists flag destination domains-the URLs spammers link to. If you include a link to a domain listed on SURBL or URIBL, your email may be blocked regardless of your own sender reputation.

These lists identify domains used in phishing, malware distribution, and spam landing pages. They're updated continuously as new malicious domains are discovered. The danger for legitimate senders comes from linking to compromised domains (a legitimate site that's been hacked), shortened URLs that redirect to listed domains, or shared infrastructure where bad actors use the same URL shorteners or content hosts you do.

Protect yourself by auditing all links before sending: check that landing page domains aren't on major content blocklists, avoid obscure URL shorteners, and use your own tracking domains rather than shared services. Be cautious with user-generated content that might contain malicious links. If you run affiliate programs, vet affiliate links carefully. You can do everything right with your own sending infrastructure and still get blocked because of one link to someone else's problematic domain-content blocklists hold you responsible for where you send recipients.