How does a CBL domain listing affect email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Aug 2025
Updated 19 Aug 2025
9 min read
A listing on the Composite Blocking List (CBL) can certainly raise concerns for anyone managing email deliverability. This real-time blacklist (also known as a blocklist) specifically targets IP addresses that are observed sending spam or exhibiting spam-like behavior. While the CBL primarily lists IP addresses, the impact can extend to the domain reputation of websites and email senders associated with those IPs.
The question of how severely a CBL listing affects email deliverability is complex. Many factors come into play, including whether the domain itself is sending email from the listed IP, the specific type of mail being sent, and how heavily recipients' email providers rely on the CBL. Understanding these nuances is crucial for diagnosing and addressing any potential deliverability issues.
I’ve seen instances where a domain's website IP is listed on the CBL due to a shared hosting environment or an infected site, even when the email sending IP is entirely different and clean. In such cases, the direct impact on email deliverability might be minimal, but it’s never zero. The association can still cause problems for certain mailbox providers that use the CBL data in their broader spam filtering mechanisms.
What is the CBL and how does it list domains?
The CBL is maintained by Spamhaus, a leading authority on email blacklists and blocklists. It identifies IP addresses that exhibit characteristics of compromised machines or systems that are sending spam or acting as sources for other malicious activity. This could be due to open proxies, infected web servers, or other forms of abuse. The listing is often real-time, meaning that as soon as a problem is detected, the IP is added to the blocklist.
One significant aspect of the CBL is its role in feeding other, broader Spamhaus blocklists, such as the Spamhaus Zen list. Zen combines several Spamhaus lists, including the CBL, Spamhaus Blocklist (SBL), and Exploits Blocklist (XBL). This means that an IP listed on the CBL will also be included in Zen, amplifying its potential impact. You can learn more about the Zen list on the Spamhaus Blocklist page.
Because the CBL focuses on IP addresses, a domain listing usually occurs because the domain's A record (which points to its web server IP) is on a compromised IP. It's not the domain itself that is directly blacklisted, but its association with an IP found to be a source of spam or malicious activity. This can be particularly frustrating for domain owners who might be caught in a shared hosting environment where another tenant is causing the issue, or if their website is compromised without their immediate knowledge.
Understanding CBL Listings
The CBL lists IPs, not domains directly, based on detected spam sources. If your domain's website is on a listed IP, it's flagged by association. This often happens with shared hosting or compromised web servers.
Direct vs. indirect impact on email deliverability
The primary impact of a CBL listing on email deliverability depends heavily on which IP address is being blocked. If your domain's web server IP is on the CBL, but your emails are sent from a completely different, clean IP address via a reputable email service provider, the direct impact on your email deliverability might be minimal. This is because most email providers primarily filter incoming mail based on the sending IP address and domain reputation established through email authentication protocols like SPF, DKIM, and DMARC. However, a blacklisted IP address associated with your domain can still raise red flags indirectly.
Mailbox providers, especially large ones like Outlook.com and other smaller receivers, often use a combination of IP blacklists (or blocklists) and domain reputation metrics to determine inbox placement. If your domain's A record points to a CBL-listed IP, some filters might perceive this as a general indicator of a compromised or problematic domain, even if your email is sent from a different, clean IP. This could lead to increased spam filtering, delayed delivery, or even outright rejection of your emails.
The true impact can often be discovered through testing. Sending test emails to seed lists that monitor deliverability across various providers can provide insights into whether your emails are landing in the inbox, spam folder, or being blocked. This can help confirm if the CBL listing, or its propagation to other blocklists like Zen, is indeed affecting your email campaigns. As highlighted by Mailgun, being listed on a blocklist doesn't always guarantee deliverability issues, but it's a significant warning sign that shouldn't be ignored.
Distinguishing IP blocklisting from domain reputation
It's important to distinguish between an IP being blacklisted and the actual impact on your email sending domain. If your emails are sent from a dedicated, clean IP address that is not on any major blocklist, and your domain's email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned, your emails are more likely to reach the inbox. The CBL listing of your web server's IP (via your domain's A record) may cause some minor issues with very aggressive spam filters, but it typically won't outright block your email if your sending infrastructure is separate and secure.
However, the domain itself still plays a crucial role in email deliverability. Mailbox providers assess domain reputation independently of the sending IP to some extent. A domain with a history of association with spammy IPs, even if it's passive (like a website IP listing), could still contribute to a tarnished reputation. This is where the indirect impact can manifest, potentially leading to emails being sent to the spam folder or facing increased scrutiny from spam filters.
The key takeaway is that while a CBL domain listing might not directly block your email traffic if your sending IP is clean, it's a symptom of a potential underlying issue (like a compromised website or shared bad neighborhood IP). Addressing this underlying issue is vital for long-term email health, especially as Google and Yahoo introduce stricter sender requirements. Ensuring your entire digital footprint, including your website and email infrastructure, maintains a clean reputation is paramount.
IP blocklist
Focus: Lists specific IP addresses identified as sending spam or hosting malicious content.
Direct impact: Can directly block email sent from that specific IP, leading to bounces or spam folder delivery.
Resolution: Requires fixing the root cause of the spam/malware and then requesting delisting from the relevant IP blacklist.
Domain associated with blocklisted IP
Focus: A domain's A record points to an IP that is on a blocklist (like CBL).
Indirect impact: If email is sent from a different, clean IP, direct blocks are less likely. However, it can still negatively affect overall domain reputation and trigger some spam filters.
Resolution: Clean up the compromised web server or move the domain to a clean IP, then request delisting for the IP. Monitoring domain reputation is key.
Remediation and prevention strategies
If your domain is associated with a CBL listing, the first step is to identify the cause. Is it your web server's IP? Is your website compromised? CBL provides details about the nature of the infection or abuse, which is crucial for remediation. You can check the specific listing details, such as those often found on Abuseat.org CBL lookup. Once the root cause is addressed (e.g., removing malware from a website, securing an open proxy), you can request delisting from the CBL. They generally offer a straightforward delisting process once the issue is resolved.
For ongoing prevention, maintaining good security practices for your web servers and ensuring your email sending infrastructure is separate and well-maintained is key. If you're using a shared hosting environment, it might be worth considering a dedicated IP for your website or emails if recurrent issues arise due to other tenants' actions. Regular blocklist checking is also important to catch problems early. If you need to resolve a domain blocklisting, consider our dedicated guide.
Ultimately, a CBL listing of your domain's associated IP is a flag that should prompt investigation. While its direct impact on email deliverability may vary, especially if your sending IPs are separate, it always indicates a need to review your web hosting environment and security practices. A clean reputation across all facets of your digital presence contributes to overall email deliverability success. More broadly, it's wise to review why emails fail and address any foundational issues.
Views from the trenches
Best practices
Always maintain a strict separation between your web hosting and email sending IPs to minimize cross-contamination of reputation.
Implement robust security measures on your web servers to prevent malware infections and compromised websites.
Regularly monitor your domain's associated IP addresses for any blocklist listings, even if they aren't your primary sending IPs.
Common pitfalls
Assuming a CBL listing on a web server IP has no impact on email deliverability, especially for smaller receivers.
Neglecting to investigate and remediate the root cause of the listing, which can lead to recurring issues or broader reputation damage.
Relying solely on shared hosting environments without considering the potential for 'bad neighbors' impacting your domain.
Expert tips
If a web server IP is listed, check if the domain's email is sent from the same IP. If not, the impact on email deliverability should be minimal, but still warrants attention.
CBL listings often propagate to other major blacklists (like Spamhaus Zen), so resolving the CBL issue can have a broader positive effect.
Ensure your DNS records, especially A records, are pointing to clean, secure IPs. This protects your domain's overall health.
Marketer view
Marketer from Email Geeks says a client's domain was listed on CBL due to a compromised website, which was ironic given their internet security provider. The listing was for the website IP, not the email sending IP.
2019-05-29 - Email Geeks
Expert view
Expert from Email Geeks says fixing the infected host name and malicious redirects should be straightforward to address the CBL listing.
2019-05-29 - Email Geeks
Final thoughts on CBL listings and deliverability
In summary, a CBL domain listing typically means that an IP address associated with your domain, most commonly your web server's IP, has been identified as a source of spam or malicious activity. While your email sending IP may be different and clean, this association can still indirectly affect your domain's reputation and lead to deliverability challenges with some mailbox providers.
The key is to understand the nature of the listing, swiftly address the underlying security or configuration issue, and request delisting. Proactive monitoring of your domain and associated IPs across various blocklists, coupled with robust email authentication and content practices, will safeguard your email deliverability and overall online presence.
Maintaining a pristine sender reputation is a continuous effort, and being aware of how various blocklists, like the CBL, operate is essential for any sender aiming for consistent inbox placement. Don't just ignore these listings, but investigate to ensure your email program remains healthy.
Outlook.com and other smaller receivers, often use a combination of IP blacklists (or blocklists) and domain reputation metrics to determine inbox placement. If your domain's A record points to a CBL-listed IP, some filters might perceive this as a general indicator of a compromised or problematic domain, even if your email is sent from a different, clean IP. This could lead to increased spam filtering, delayed delivery, or even outright rejection of your emails.
Google and 
Yahoo introduce stricter sender requirements. Ensuring your entire digital footprint, including your website and email infrastructure, maintains a clean reputation is paramount.
