Summary
The Composite Blocking List, or CBL, primarily targets IP addresses identified as sources of spam or malicious traffic. While it doesn't directly list domains, a CBL listing for an IP address your domain uses for sending emails will severely impact your email deliverability. This is because many email servers, including those of major providers like Gmail and Outlook.com, extensively use CBL data to filter incoming mail, leading to immediate rejections, blocks, or routing to junk folders. Such listings significantly damage your domain's sender reputation and often indicate a compromised server, application, or an issue within shared sending infrastructure. Resolving the underlying cause of the listing is crucial to restore normal email flow and avoid widespread rejections, often accompanied by specific '550 blocked by CBL' error messages.
Key findings
- IP-Based Listing: The CBL is primarily an IP-based real-time blacklist, meaning it lists specific IP addresses observed sending spam or malicious traffic, rather than directly listing domains.
- Severe Deliverability Impact: If your domain's sending IP is listed on the CBL, emails are highly likely to be rejected, blocked, or routed to spam folders by a wide array of recipient mail servers and email providers, including major ones like Outlook.com, Gmail, and services like Mimecast.
- Damaged Sender Reputation: A CBL listing severely damages the associated domain's sender reputation, which can affect all future email attempts even after the immediate issue is resolved.
- Broad Propagation: CBL data often feeds into other widely-used blacklists, such as the Zen list and XBL, amplifying the negative impact and broadening the scope of email rejection across various email providers.
- Indicates Compromise: CBL listings frequently signal a compromised server, infected hostname, application vulnerability, or an issue within shared sending infrastructure that is generating spam or malicious traffic.
Key considerations
- Verify Sending IP: Crucially, determine if the specific IP address from which your domain sends emails is the one listed on the CBL. A listing of your domain's A record on the CBL while your actual mail is sent from a different, clean IP, might result in minimal to zero impact on email deliverability.
- Prompt Resolution: A CBL listing indicates an urgent underlying issue, such as a compromised server or active spamming, that requires immediate investigation and resolution to restore email deliverability.
- Monitor for Bounce Codes: Regularly monitor your bounce messages for specific errors like '550 blocked by CBL,' as these directly confirm that the CBL is impacting your email delivery.
- Implement Good Sending Habits: Practice consistent good sending habits, including robust server security and vigilant monitoring for suspicious activity, to prevent future CBL listings and maintain a healthy sender reputation.
What email marketers say
11 marketer opinions
A CBL listing, which targets the IP address your domain uses for sending emails, has a profound and immediate negative impact on deliverability. When this occurs, recipient mail servers, including those of major providers like Gmail and Outlook, and security services such as Mimecast, are highly likely to block your emails or direct them to spam folders. This widespread rejection severely damages your sender reputation, resulting in high bounce rates and poor campaign performance. Such a listing often signals a significant underlying issue, like a compromised server, application, or misconfiguration that is actively sending spam, necessitating urgent resolution.
Key opinions
- Immediate Rejection and Blocking: Emails sent from a CBL-listed IP address face immediate and widespread rejection or blocking by a diverse range of mail servers and security platforms, including major providers.
- Severe Reputation Harm: A CBL listing severely damages the domain's sender reputation, leading to high bounce rates and poor email campaign performance, impacting future sending attempts.
- Indicates Security Compromise: The presence of an IP on the CBL often points to a compromised server, an infected host, or a misconfigured application actively engaged in sending spam or malicious traffic.
- Feeds Other Blacklists: The CBL's real-time data can propagate to other widely-used blacklists, such as XBL and Zen, further amplifying the negative effect on deliverability across the internet.
- Compromise Requires Cleanup: Restoring email flow after a CBL listing typically requires a thorough cleanup of the compromised server or resolution of the underlying misconfiguration that caused the spamming activity.
Key considerations
- Identify Actual Sending IP: It is crucial to confirm whether the specific IP address used for sending your domain's email is the one listed on the CBL, as a listing of a non-sending IP, like a Cloudflare shared IP for a website, may have minimal or no direct impact on email deliverability.
- Prioritize Root Cause Resolution: A CBL listing is a critical alert for an active issue, demanding immediate investigation and resolution of the underlying cause, whether it is a security breach or a configuration error.
- Maintain Good Sending Hygiene: Consistent practice of good sending habits, including secure server management and vigilant monitoring for unauthorized activity, is vital to prevent future CBL listings and sustain a strong sender reputation.
- Monitor Bounce Messages for Codes: Regularly checking bounce messages for specific codes indicating a CBL block, such as '550 blocked by CBL', can confirm the exact reason for non-delivery and guide remediation efforts.
Marketer view
Marketer from Email Geeks explains that a CBL listing, especially if it propagates to XBL and then to the widely-used ZEN blacklist, can have a huge impact on email deliverability, specifically affecting performance at receivers like Outlook.com and many smaller ones.
9 Mar 2022 - Email Geeks
Marketer view
Marketer from Email Geeks explains that if email is not being sent from the CBL-listed IP address, typically a shared IP like those from Cloudflare, the impact on email delivery from the domain's A record listing will likely be zero to minimal. He advises clients to get off Cloudflare on principle, but reassures that the specific CBL domain listing isn't likely to significantly affect email deliverability if the mail sending IP is clean.
10 Jan 2022 - Email Geeks
What the experts say
3 expert opinions
While the Composite Blocking List (CBL) directly targets IP addresses, its impact on email deliverability for a domain is direct and severe if that domain's sending IP is listed. This commonly indicates an underlying issue, such as a compromised host or problem within shared infrastructure, that is actively sending spam. Mail servers frequently reject emails originating from CBL-listed IPs, leading to significant delivery failures and a widespread inability for your domain's emails to reach recipients.
Key opinions
- IP-Based System: The CBL operates by listing IP addresses, not domain names, that have been identified as sources of spam.
- Severe Deliverability Impact: When a domain's outbound mail utilizes an IP address listed on the CBL, recipient mail servers will extensively reject or block emails, causing widespread delivery failures.
- Indicates Compromise: A CBL listing often signifies a compromised system, such as an infected hostname, or an issue within shared sending infrastructure where a client is compromised and sending spam.
- Wider Blacklist Impact: Data from the CBL feeds into broader blacklists, like the Zen list, extending the negative impact of a listing across a wider array of email filtering systems.
Key considerations
- Root Cause Resolution: It is imperative to identify and resolve the underlying issue that led to the CBL listing, whether it's a compromised server, an infected machine, or a problem within shared infrastructure, to ensure long-term deliverability.
- Immediate Action Required: Given the real-time nature and severe impact of CBL listings, immediate investigation and remediation are necessary to restore normal email flow and prevent sustained deliverability issues.
- Monitor Sending Infrastructure: Regular monitoring of your email sending IPs and infrastructure for unusual activity or signs of compromise is crucial to prevent CBL listings.
Expert view
Expert from Email Geeks explains that CBL listings often point to an infected hostname or indicate an issue within shared infrastructure where another client is compromised. He notes that the CBL feeds into the Zen list, suggesting a mechanism for wider blacklist impact.
28 Jun 2024 - Email Geeks
Expert view
Expert from Spam Resource explains that the Composite Blocking List (CBL) is an IP-based real-time blacklist of IP addresses detected as sending spam. While CBL directly lists IPs, if a domain's sending IP is listed on the CBL, mail servers will likely reject emails originating from that IP, thus severely impacting the deliverability of all emails sent from that domain.
14 Aug 2021 - Spam Resource
What the documentation says
5 technical articles
A Composite Blocking List (CBL) entry, while targeting the IP address used by your domain for email sending, immediately and severely undermines email deliverability. This translates into widespread email rejection, quarantining, or automatic routing to junk folders by recipient mail servers. ISPs and email providers widely utilize CBL data for filtering, which means a listing often results in high bounce rates and specific non-delivery error messages, such as '550 blocked by CBL.' Ultimately, this listing signals that the sending IP for your domain has been implicated in sending spam or malicious traffic, directly affecting your domain's ability to reach inboxes and damaging its sender reputation.
Key findings
- IP-Targeted, Domain-Affected: The CBL is an IP-based blacklist; however, if your domain uses a listed IP, its email deliverability is directly and negatively impacted.
- Widespread Rejection: Emails sent from a CBL-listed IP are widely rejected, quarantined, or sent to spam folders by email servers that rely on CBL data, leading to severe delivery failures.
- Reputation Damage: A CBL listing immediately damages the sender reputation of the associated domain, which can lead to continued deliverability issues even after the initial problem is addressed.
- Error Message Indicators: Often, a CBL block is indicated by specific error messages, such as '550 blocked by CBL,' providing clear feedback on the cause of non-delivery.
- Spam Implication: A CBL listing signifies that the IP address sending emails for your domain has been involved in sending spam or malicious traffic, prompting recipient servers to treat your mail as undesirable.
Key considerations
- Confirm Sending IP's Status: Verify that the specific IP address your domain uses for sending emails is indeed the one listed on the CBL, as this confirms the direct cause of deliverability issues.
- Address Spam Source Immediately: A CBL listing necessitates immediate investigation and remediation of the root cause that led to the IP sending spam or malicious traffic, which is critical for restoring email flow.
- Monitor for Bounce Notifications: Pay close attention to bounce messages for specific codes like '550 blocked by CBL,' as these provide crucial diagnostic information for addressing the listing.
- Proactive Deliverability Management: Implement continuous monitoring and adhere to best practices for email sending to prevent future CBL listings and maintain a robust sender reputation.
Technical article
Documentation from Spamhaus.org explains that the CBL primarily lists IP addresses observed sending spam or malicious traffic. If an IP address your domain uses is listed on the CBL, email sent from that IP will likely be rejected or routed to junk folders by many email servers that use CBL data, thereby directly impacting the deliverability of emails sent under your domain.
29 Jun 2023 - Spamhaus.org
Technical article
Documentation from Validity explains that although the CBL targets IP addresses, a listing directly and negatively impacts the associated domain's sender reputation and deliverability because many Internet Service Providers (ISPs) rely on CBL data for filtering, resulting in high bounce rates and unwanted spam folder placement.
6 Nov 2022 - Validity.com
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