What is a "URI-based" blocklist and how is it triggered?

URI-based blocklists (also called URL blocklists or domain blocklists) filter email based on the links contained within the message body rather than the sending IP. Even mail from perfectly reputable senders will be filtered if it contains links to blocklisted domains.

How URI blocklists work:

1. Email arrives and passes IP reputation checks
2. Content filter extracts all URLs/domains from the message body
3. Each domain is checked against URI blocklists (SURBL, URIBL, Spamhaus DBL, etc.)
4. If any domain is listed, the email is filtered or scored accordingly

What triggers URI blocklist entries:

• Malware/phishing distribution: Domains hosting or redirecting to malicious content
• Spam link frequency: Domains appearing repeatedly in spam messages
• Affiliate/tracking links: Services commonly used by spammers
• URL shorteners: Shortening services used to mask spam destinations
• Newly registered domains: Fresh domains often indicate disposable spam infrastructure
• Redirector abuse: Domains used in redirect chains to hide final destinations

Common URI blocklists:

• SURBL - Multi-source URI blocklist
• URIBL - SpamAssassin-integrated URI checking
• Spamhaus DBL - Domain blocklist from Spamhaus
• Google Safe Browsing - Used by Gmail and Chrome

How to avoid URI blocklist issues:

• Scan all links before sending with tools like Mail-Tester
• Avoid URL shorteners in marketing email
• Check third-party tracking domains you integrate
• Monitor domains in your email templates, not just your sending domain
• Be cautious with affiliate links or user-generated content

URI blocklists watch the destinations you're pointing people toward. Your ship may be legitimate, but if you're handing out maps to pirate coves, you'll be treated as an accomplice.