What should I do if my IP address is listed in CBL?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 Jul 2025
Updated 15 Aug 2025
7 min read
Finding your IP address on the Composite Blocking List (CBL) can be a significant setback for your email deliverability. This particular blacklist (or blocklist) is maintained by Spamhaus and primarily lists IP addresses that show signs of being compromised by malware, viruses, or acting as open proxies or botnet components. Unlike traditional spam blocklists, a CBL listing indicates a deeper security issue that requires immediate attention, as it can severely impact your ability to send emails and even affect website access.
When your IP is listed, emails from that IP are likely to be rejected or routed directly to spam folders by major email providers like Gmail and Yahoo. This can lead to missed communications, lost business, and damage to your sender reputation. Addressing a CBL listing is not just about getting off a blocklist, it's about securing your infrastructure from potential threats.
Understanding your CBL listing
The first step is to confirm the listing and understand its nature. You can use a blocklist checker to see if your IP is indeed on the CBL (Composite Blocking List). Once confirmed, visiting the CBL IP address lookup page will provide specific details about why your IP was listed. This information is crucial for pinpointing the exact problem.
Unlike many other blocklists that track spam, the CBL (Composite Blocking List) focuses on IP addresses that are actively compromised and emitting malicious traffic. This could include botnet infections, spam sending trojans, or open proxies. Understanding this distinction is key to effective remediation. Your domain might be clean, but if the IP address it's associated with is compromised, your email deliverability will suffer.
Keep in mind that if your IP is listed on the CBL, it's often a precursor to being listed on the Spamhaus Exploits Blocklist (XBL). The Spamhaus XBL is a real-time blocklist of hijacked computers infected with viruses and other malware, commonly used by internet service providers and corporations to filter out hostile traffic. Knowing the exact reason for the CBL listing from their lookup tool will guide your next steps.
CBL is not a spam list
The CBL (Composite Blocking List) is designed to list IP addresses that exhibit characteristics of being infected with malware or participating in other malicious activities, such as acting as a botnet or open proxy. This is distinct from blocklists that primarily target the sending of unsolicited bulk email (spam).
Immediate actions for a CBL listing
The most critical action is to stop any malicious activity originating from the compromised IP. If the IP address is actively sending out hostile mail, this must cease immediately. This might involve temporarily stopping all email sending from the affected IP or server until the root cause is identified and remediated. Continuing to send while listed will only deepen the problem and make delisting harder.
Identifying the source of the infection is paramount. This could be a compromised server, a web host that is infected, or even a workstation within your network if it's a dynamic IP listing. You need to verify if the IP address belongs to your email service provider (ESP) or your own infrastructure (e.g., a web server). If it's your ESP's shared IP, you'll need to work with them to resolve it. If it's your own, the responsibility falls on you or your IT team.
Inspect your network for unusual outbound traffic. Tools that monitor network activity can help you identify rogue connections or unexpected data streams that might indicate a compromise. Look for activity on unusual ports or large volumes of traffic that don't correspond to legitimate operations. This forensic work is vital for effective remediation.
Checking for unusual outbound SMTP traffic on WindowsBASH
netstat -ano | findstr ":25"
Checking for unusual outbound SMTP traffic on Linux/macOSBASH
lsof -i :25
Diagnosing and resolving the underlying issue
Once you've stopped the malicious activity, the next crucial step is to thoroughly diagnose and eliminate the root cause of the infection. This is often the most challenging part, as malware can be deeply embedded. It's not enough to simply request delisting, the problem must be fixed or your IP will quickly be relisted on the CBL (Composite Blocking List) or other blacklists (blocklists).
This process involves a deep scan of all systems associated with the listed IP, including web servers, email servers, and any connected devices behind a firewall. Look for vulnerabilities like outdated software, weak passwords, or misconfigured open proxies. Review server logs, firewall logs, and mail logs for suspicious activity, unusual logins, or uncharacteristic outbound connections. The causes of CBL listings often point to security breaches.
If you are using a shared IP address, the compromise might originate from another user on the same IP. In such cases, you need to contact your ESP or hosting provider immediately to report the issue and request their intervention. They are responsible for maintaining the security of their shared IP ranges. If you're on a dedicated IP, the problem is directly within your control.
Common infection sources
Compromised servers: Web servers, email servers, or other hosting environments infected with malware.
Botnet infections: Client machines (desktops, laptops) within your network controlled by malicious actors.
Open proxies/relays: Misconfigured systems allowing unauthorized third-party mail relay.
Signs of compromise
High outbound port 25 traffic: Unexpected volumes of email-related traffic.
System slowdowns: Performance issues due to malicious background processes.
Requesting delisting and long-term prevention
After you have successfully removed the malware or fixed the vulnerability, you can request delisting from the CBL (Composite Blocking List). The process is generally straightforward. Visit the CBL removal page, enter your IP address, and confirm that the issue has been resolved. CBL usually delists IPs quickly once they detect the problem has ceased, but it's vital to have fixed the root cause, or you'll find yourself back on the blocklist.
To prevent future listings, implement robust security measures. This includes regularly updating all software, using strong, unique passwords, and deploying firewalls. Consider using an email authentication protocol like DMARC, SPF, and DKIM to further secure your email sending. Regular security audits and continuous blocklist monitoring are also essential.
If you are concerned about your email deliverability while resolving a CBL (Composite Blocking List) listing, consider temporarily switching to a different email sending infrastructure, perhaps with a new ESP, and initiating an IP warmup process. This allows you to maintain email flow while you diligently work on cleaning and securing the original IP address. You can also send from a subdomain while you resolve the issue on the main domain.
Best practices for prevention
Regular security audits: Periodically scan your systems for vulnerabilities and malware.
Software updates: Keep all operating systems, applications, and server software patched.
Strong authentication: Use complex passwords and multi-factor authentication where possible.
Network segmentation: Isolate critical systems to limit potential spread of infection.
Monitor outbound traffic: Continuously monitor for unexpected or high-volume connections.
Views from the trenches
Best practices
Always verify the exact reason for the CBL (Composite Blocking List) listing on their lookup page.
Prioritize securing the compromised system before attempting any delisting requests.
Implement continuous monitoring for outbound traffic and blocklist status.
Regularly update all software and enforce strong authentication practices across your network.
Common pitfalls
Requesting delisting without fully resolving the underlying malware infection.
Ignoring the CBL listing, assuming it's just a 'spam' blocklist that will resolve itself.
Failing to investigate all possible sources of compromise, including internal workstations.
Not communicating with your ESP or hosting provider if a shared IP is listed.
Expert tips
A CBL (Composite Blocking List) listing is often an indicator of a severe security breach, not just a spam issue.
If your IP is listed multiple times, it strongly suggests an ongoing infection that has not been fully eradicated.
Consider a temporary IP or ESP change for critical email streams while extensive remediation is performed.
Ensure your DNS records, especially SPF and PTR, are correctly configured for any IP changes.
Marketer view
My client is experiencing severe email deliverability issues, with their IP address listed on Spamhaus Zen and other blacklists, including CBL, despite their domain not being listed on any DBL.
2020-12-01 - Email Geeks
Expert view
An IP listed on CBL indicates it is likely infected with malware and is sending out hostile mail.
2020-12-01 - Email Geeks
Restoring your email reputation
Dealing with a CBL (Composite Blocking List) listing can be daunting, but it’s a clear signal that your email infrastructure's security needs immediate attention. Unlike typical spam issues, a CBL listing points to a deeper compromise, such as malware or an open proxy, that could be actively harming your sender reputation and impacting all email communications.
By understanding the specific cause of the listing, taking immediate action to stop malicious traffic, thoroughly eradicating the root cause, and implementing strong preventative measures, you can restore your IP's reputation and ensure your emails reach their intended inboxes. Proactive security and consistent monitoring are your best defenses against future blocklist challenges.
Gmail and 
Yahoo. This can lead to missed communications, lost business, and damage to your sender reputation. Addressing a CBL listing is not just about getting off a blocklist, it's about securing your infrastructure from potential threats.
