How to troubleshoot a SURBL or blocklist listing for shared email infrastructure?
Matthew Whittaker
Co-founder & CTO, Suped
Published 31 May 2025
Updated 19 Aug 2025
5 min read
Dealing with a SURBL or other blocklist listing when you're using shared email infrastructure can be a particularly tricky situation. It's not always immediately clear whether the issue stems from your own sending practices or from another user sharing the same resources. This ambiguity often complicates the troubleshooting and resolution process.
The challenge is amplified because shared infrastructure means your email reputation is tied to the actions of others. A single bad actor on a shared IP address or using a shared tracking domain can lead to your legitimate emails being blocked or flagged as spam, even if your practices are impeccable. My goal here is to help you navigate this complex landscape.
Understanding what caused the listing and how to address it quickly is paramount to restoring your email deliverability. I'll walk you through the steps to identify the problem, troubleshoot the specific type of listing, and work towards a swift resolution, particularly focusing on the nuances of shared environments.
Understanding shared infrastructure and blocklists
Shared email infrastructure typically means multiple senders use the same IP addresses, sending domains, or tracking domains. This setup is common with email service providers (ESPs) and shared hosting environments, where cost-effectiveness and ease of use are priorities. While convenient, it introduces a shared reputation model.
Blocklists (also known as blacklists) are databases of IP addresses or domains identified as sources of spam or malicious content. They are a critical component of anti-spam efforts, helping email providers filter out unwanted messages. When an IP or domain is listed, emails originating from or linking to it can be blocked or heavily scrutinized.
There are different types of email blocklists. Some are IP-based (RBLs or DNSBLs), which list sender IP addresses. Others, like SURBL (Spam URI Realtime Blocklists), are URL-based, focusing on URLs found within email bodies. Understanding this distinction is key to effective troubleshooting, especially when multiple entities share infrastructure.
Identifying the source of the listing
The first step in any blocklist (or blacklist) issue is to confirm the listing and identify which specific list you're on. You can usually do this by checking the bounce message you receive, which often contains details about the blocking entity. You can also use a blocklist checker by entering your sending IP address or domain.
How to check for a listing
When troubleshooting a listing on shared infrastructure, it's crucial to determine if the listed item is your sending IP, a shared tracking domain, or a DKIM signing domain. Sometimes, a shared view in browser link might be the culprit if a client hasn't configured a custom CNAME. Check your email headers for any domains or IPs that aren't directly yours but are part of the email's journey.
Aspect
IP-based blocklist (e.g., Spamhaus SBL)
URL-based blocklist (e.g., SURBL)
Primary target
Sending IP address of the mail server
URLs (domains) found within the email's body
Cause of listing
Sending unsolicited email, high bounce rates, spam complaints from that IP
Linking to known spam, phishing, or malware sites. Shared tracking domains linking to problematic content.
Troubleshooting focus
Reviewing IP reputation, sender behavior, and list hygiene for all users on the shared IP
Identifying and removing problematic URLs in email content or tracking redirects across all shared clients
Once you've narrowed down the type of blocklist, you can begin to investigate the specific cause. For shared infrastructure, this often involves collaboration with your ESP or hosting provider, as they have insight into other users' activities on the shared resources. I've found that pinpointing the exact offending element is the hardest part.
Steps to troubleshoot a SURBL listing
SURBLs are unique because they don't list IP addresses, but rather domains (URLs) found in the body of spam messages. This means that even if your sending IP is clean, a malicious or spammy URL in your email content, or in a shared tracking domain, can trigger a SURBL listing. This is particularly relevant for shared infrastructure where multiple clients might use the same default tracking domains.
If you suspect a SURBL (or other URL-based blocklist) listing, you need to check the URLs present in your outgoing emails. This includes not just the visible links, but also hidden tracking links (like click tracking or open tracking pixels), and especially any default domains your ESP uses for these purposes. You can use the SURBL lookup page to verify a listing.
Example of a shared tracking URL
https://sharedtrack.example.com/click?id=123
To resolve a SURBL listing, first identify the specific URL (or URLs) that caused the problem. Once identified, you must remove or clean up the harmful content associated with that URL. If it's a shared domain, you'll need to work with your provider to ensure the issue is addressed for all users, or switch to a dedicated tracking domain. After rectifying the problem, follow the instructions on the SURBL website for removal requests. Learn more about how SURBL impacts deliverability.
Addressing general blocklist listings on shared infrastructure
When you're on shared infrastructure, the actions of one user can impact everyone. If another client on your shared IP or using a shared domain sends spam, engages in phishing, or has poor list hygiene, your email deliverability can suffer. This is why understanding your shared environment is so important.
Proactive measures
Monitor your reputation: Regularly check your sending IP and domains on common blocklists. This proactive step helps you detect issues early.
Maintain list hygiene: Clean your email lists regularly to remove inactive subscribers, typos, and spam traps.
Use proper authentication: Ensure your emails are properly authenticated with SPF, DKIM, and DMARC records to build sender trust.
Reactive measures
Contact your provider: If the listing is on a shared IP, your ESP or hosting provider is usually responsible for the delisting process. Manage senders and identify the cause.
Address the root cause: Whether it's a compromised account, poor sending practices by another user, or a specific URL, the issue must be fixed before a delisting request will be successful.
Request delisting: Once the problem is resolved, submit a delisting request to the specific blocklist operator. Be honest and provide all requested information.
Remember that IP-based blocklists will impact all users on that shared IP, while URL-based blocklists will affect all emails containing the listed URL, regardless of the sender's IP. This means you might be affected by issues beyond your direct control. Understanding what happens when your IP gets blocklisted is critical.
Views from the trenches
Best practices
Clearly communicate with your email service provider about any blocklist issues.
Ensure all URLs in your email content, including tracking links, are clean and legitimate.
Regularly monitor your shared IP and domain reputation for any red flags.
Implement strong sender authentication standards like SPF, DKIM, and DMARC.
Common pitfalls
Assuming the problem is solely your fault without checking for shared infrastructure issues.
Ignoring warning signs in bounce messages or deliverability reports.
Failing to clean your email lists, leading to higher bounce rates and spam complaints.
Not understanding the difference between IP-based and URL-based blocklists.
Expert tips
If using a shared DKIM signing domain, consider if a custom DKIM setup for your clients could help isolate reputation.
Remember that SURBL primarily focuses on URLs in the body content, not necessarily headers or sending IPs.
A shared view-in-browser link or a link redirect URL can often be the source of a SURBL listing if not properly managed or customized by clients.
For shared IPs, collaborate closely with your provider, as they are often the only ones who can initiate a delisting.
Marketer view
A marketer from Email Geeks says they found their DKIM signing domain on a blocklist, which was shared among multiple clients.
2018-05-10 - Email Geeks
Expert view
An expert from Email Geeks says that SURBL typically focuses on URLs in email body content, not necessarily URLs found in headers.
2018-05-10 - Email Geeks
Wrapping up
Troubleshooting SURBL or other blocklist listings on shared email infrastructure demands a clear understanding of the blocklist type, a meticulous investigation into the offending URLs or IPs, and often, close collaboration with your email service provider. While frustrating, it's a manageable challenge with the right approach.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
