Guide

SURBL

Overview

SURBL (Spam URI Realtime Blocklist) is a real-time database that tracks the reputation of domains and URIs found within email bodies worldwide. Unlike traditional DNSBLs that focus on the sending IP address, SURBL focuses on the content inside the email—specifically links and URLs.

SURBL is not a single list but a collection of specialized databases designed to categorize different types of threats: phishing (PH), malware (MW), cracked sites (CR), abuse (AB), and a combined "Multi" list that allows mail servers to query all sub-lists in a single DNS lookup.

Impacts

This listing occurs at the domain/URI level (not IP level). When your domain is listed on SURBL, emails containing links to your domain may be rejected, filtered, or marked as spam by mail servers that query SURBL data. This can result in "silent" delivery failures or specific bounce-back codes like "Message Content Rejected" (554 Error).

Major ISPs like Gmail and Outlook use SURBL data to disable links in the inbox, making them unclickable. This directly impacts click-through rates and email campaign effectiveness.

Mitigation Process

To better understand the potential problems that caused the listing, review the SURBL website. Once you have reviewed, you should then:

  1. Find and correct the root cause of the listing (e.g., compromised website, phishing pages, malware distribution, or spam-related content).
  2. Visit the SURBL Lookup page to check your domain status and follow the instructions for removal.
  3. Submit a removal request through the SURBL removal form. You must demonstrate that the underlying issue has been resolved.
  4. Wait for the review process. SURBL uses a human-machine hybrid approach with automated honeypots combined with manual verification by security researchers.

Important: The issue must be resolved prior to requesting removal. SURBL is particularly strict about cracked sites (hacked legitimate websites). If your site was compromised, you must thoroughly clean it and implement security measures to prevent re-infection. If you delist but the site remains compromised, you will be re-listed immediately.