Summary
An IP address is listed on the CBL primarily due to compromised systems (malware infections, bot-like behavior, open proxies, compromised credentials), spamming activities (sending unsolicited emails, hitting spam traps, poor list hygiene), and exploited web server vulnerabilities. These issues can cause spam outbreaks and negatively impact IP reputation, ultimately leading to blacklisting. Resolution strategies encompass identifying and resolving the source of the problem (e.g., removing malware, patching vulnerabilities), improving sending practices (email authentication, list cleaning, respecting unsubscribe requests), monitoring IP reputation and outbound traffic for anomalies, and requesting delisting from the CBL. In some cases, the CBL listing may self-resolve once the abusive traffic ceases. Being listed on the CBL often results in emails being rejected by receiving mail servers.
Key findings
- Compromised Systems: Malware infections, compromised credentials, and bot-like behavior are primary drivers of CBL listings.
- Spamming Activities: Sending to spam traps, having poor email list hygiene, and engaging in unsolicited email campaigns lead to blacklisting.
- Web Server Vulnerabilities: Unpatched web server vulnerabilities can be exploited to send spam and cause IP blacklisting.
- Reputation Impact: CBL listings result in email rejections and damage IP reputation.
- Self-Resolution: The CBL automatically delists IPs once the abusive traffic stops, although manual intervention may be required in some cases.
Key considerations
- Immediate Action: Promptly identify and stop any activities causing the blacklisting.
- Security Measures: Implement strong passwords, two-factor authentication, and regularly scan systems for malware and vulnerabilities.
- Email Authentication: Implement SPF, DKIM, and DMARC to authenticate the sending domain.
- List Hygiene: Maintain clean email lists by removing invalid addresses and spam traps.
- Traffic Monitoring: Monitor outbound email traffic for unusual patterns and anomalies.
- Reputation Management: Monitor IP reputation and take immediate action if listed on major blocklists.
- Respect Unsubscribes: Honor unsubscribe requests promptly to avoid complaints.
- Request Delisting: Request delisting from the CBL after resolving the issues causing the listing.
What email marketers say
10 marketer opinions
An IP address can be listed on the CBL (Composite Blocking List) due to various reasons related to sending unsolicited or malicious email. These reasons include compromised credentials, malware infections, spam-like content, hitting spam traps, poor sending practices (such as high volume without proper authentication), unpatched web server vulnerabilities, and compromised email accounts. Resolution involves identifying and removing the source of the problem, securing systems, improving sending practices, cleaning email lists, implementing strong authentication, and requesting delisting from the CBL.
Key opinions
- Compromised Systems: Malware infections and compromised credentials are major causes of CBL listings.
- Poor Sending Practices: Sending to spam traps, invalid addresses, and high email volume without authentication contribute to blacklisting.
- Vulnerabilities: Unpatched web server vulnerabilities can be exploited to send spam.
- List Hygiene: Poor email list hygiene (invalid addresses, spam traps) leads to blacklisting.
Key considerations
- Security Scans: Regularly scan systems for malware and vulnerabilities.
- Account Security: Implement strong passwords and two-factor authentication.
- List Cleaning: Use a reputable email list cleaning service to remove invalid addresses and spam traps.
- Authentication: Implement SPF, DKIM, and DMARC to authenticate your sending domain.
- Monitor Reputation: Monitor IP and domain reputation to identify and address blacklisting issues promptly.
- Proactive Prevention: Regularly audit email sending practices to prevent future blacklisting
Marketer view
Email marketer from SparkPost shares that CBL listings often stem from compromised devices or malware infections sending unsolicited emails. To resolve this, they recommend identifying and removing the source of the spam, then requesting delisting through the CBL's website.
6 Feb 2024 - SparkPost
Marketer view
Marketer from Email Geeks shares a reply from John Levine explaining that nothing is infected and that the issue was caused by a random survey of .org domain web pages hitting a C&C sinkhole, leading to a false alarm listing.
6 Jul 2022 - Email Geeks
What the experts say
6 expert opinions
Being listed on the CBL often indicates that a system is compromised and sending spam. Resolution involves identifying and shutting down the source of spam, often requiring fixing an infection. Monitoring IP reputation and outbound email traffic for unusual patterns is crucial. If the issue isn't directly manageable, informing relevant parties for resolution is advised.
Key opinions
- Compromised Systems: CBL listings often result from infected systems sending spam.
- Self Resolution: Listings should self-resolve once the spam source is stopped.
- Reputation Monitoring: Monitoring IP reputation helps identify and address blacklisting issues.
Key considerations
- Identify Source: Pinpoint the source of spam (e.g., infected machine).
- Address Infection: Fix any infections causing the spam.
- Monitor Traffic: Monitor outbound email traffic for anomalies.
- Inform Relevant Parties: Contact relevant parties (e.g., network owner) if you cannot directly resolve the issue.
- Take Action: Take immediate action when your IP is found in major blocklists.
Expert view
Expert from Email Geeks shares that the system is infected with something and it’s listed on the CBL and the user needs to fix whatever is infected.
19 Jul 2021 - Email Geeks
Expert view
Expert from Word to the Wise emphasizes the importance of monitoring your IP reputation and promptly addressing any issues to avoid prolonged blacklisting. Regularly check if your IP is listed on major blocklists and take immediate action if found.
21 Sep 2024 - Word to the Wise
What the documentation says
5 technical articles
IP addresses get listed on the CBL and other blocklists primarily due to spamming activities, malware infections, botnet involvement, or exhibiting bot-like behavior (e.g., open proxies or aggressive network scanning). Exploited machines sending unsolicited email will also trigger listings. Resolution often involves stopping the abusive traffic, which can lead to automatic delisting within hours. To avoid blacklisting, implement proper email authentication (SPF, DKIM, DMARC), maintain clean mailing lists, monitor IP reputation, and respect unsubscribe requests. Being listed on the CBL often results in email rejections by receiving mail servers.
Key findings
- Spamming Activities: Involvement in spamming activities is a primary cause of IP blacklisting.
- Malware and Botnets: Hosting malware or being part of a botnet leads to IP listings.
- Automatic Delisting: The CBL typically delists IPs automatically after abusive traffic stops.
- Email Rejections: CBL listings often result in emails being rejected by recipient servers.
Key considerations
- Stop Abusive Traffic: Immediately stop any activities causing the blacklisting.
- Implement Authentication: Ensure proper email authentication using SPF, DKIM, and DMARC.
- Maintain Clean Lists: Keep mailing lists clean and up-to-date.
- Monitor Reputation: Monitor IP reputation regularly to identify and address issues.
- Respect Unsubscribes: Honor unsubscribe requests promptly.
- Check RBL Status: Check your IP status across multiple RBLs (e.g., using MultiRBL) to be aware of any listings.
Technical article
Documentation from Barracuda Networks shares that to avoid blacklisting, ensure proper email authentication (SPF, DKIM, DMARC), maintain clean mailing lists, and monitor your IP reputation. Avoid sending unsolicited emails and respect unsubscribe requests.
7 Nov 2024 - Barracuda Networks
Technical article
Documentation from MultiRBL shares that listings on the CBL will often result in emails being rejected by receiving mail servers. MultiRBL is used to check an IP's status across many different RBLs.
15 Apr 2025 - MultiRBL
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