Summary
The bounce message IP_IN_CIDR signifies that the sending IP address is part of a Classless Inter-Domain Routing (CIDR) block that has been blacklisted, typically due to a history of spam, policy violations, or other unwanted traffic originating from that range. Receiving mail servers reject connections from any IP within these blocked CIDR ranges to protect their users. Resolving IP_IN_CIDR involves identifying the specific blacklist, contacting the operator for delisting, and ensuring proper email authentication (SPF, DKIM, DMARC). Maintaining good sending practices and monitoring your IP's reputation are also crucial.
Key findings
- Root Cause: IP_IN_CIDR indicates the sending IP belongs to a blacklisted CIDR block, often due to spam or policy violations from within that range.
- ISP Influence: Using an ISP with poor IP ranges or being on a shared IP space can increase the likelihood of encountering this error.
- Blocklist Impact: The Spamhaus Block List (SBL), while not directly causing IP_IN_CIDR, can contribute to it if the sender's CIDR range is listed.
- Reputation: Even if an IP isn't actively sending spam, it gets blocked because of the overall reputation of the CIDR block it belongs to.
Key considerations
- Identify and Delist: Determine which blocklist is causing the issue and contact the blocklist operator to request delisting, demonstrating a commitment to preventing spam.
- Email Authentication: Implement or verify proper email authentication methods like SPF, DKIM, and DMARC.
- Reputation Monitoring: Use tools like Google Postmaster Tools to monitor your sending reputation and identify any issues that could lead to blacklisting.
- Best Practices: Adhere to email best practices, including maintaining a clean email list and avoiding spam triggers.
- IP Change (Extreme): Consider changing IP addresses only as a last resort if the current IP's reputation is irreparably damaged.
- Cloud Provider Review: If using a cloud provider, ensure their email infrastructure is well-maintained to avoid inheriting poor IP ranges.
- Whitelisting Request: Contact the recipient's email administrator to request whitelisting of your IP address.
What email marketers say
9 marketer opinions
The bounce message IP_IN_CIDR indicates that the sending IP address is part of a CIDR block that has been blacklisted due to a history of spam or other policy violations. This means the receiving server is configured to reject connections from any IP within that CIDR range. Even if a specific IP in the blocked range is not sending spam, it will still be rejected due to the overall reputation of the CIDR block.
Key opinions
- Cause: The primary cause is spam activity originating from the IP range, leading to blacklisting of the entire CIDR block.
- MX Records: Some domains might use MX records that point to a broadband ISP's mail server, potentially leading to issues if that ISP's IP range is blacklisted.
- Blacklist Checks: It is important to regularly check if your IP or CIDR range is listed on any public blacklists (RBLs) using tools like MXToolbox or MultiRBL.
- Cloud Providers: If using a cloud provider, ensure their email infrastructure is well-maintained, as some providers might have poor IP ranges with a history of spam issues.
Key considerations
- Delisting: If blacklisted, attempt to delist by contacting the blacklist operator and demonstrating steps to prevent spam.
- Whitelisting: Contact the recipient's email administrator to request whitelisting of your IP address.
- Email Best Practices: Ensure adherence to email best practices, including proper authentication (SPF, DKIM, DMARC), maintaining a clean email list, and avoiding spam triggers.
- Reputation Monitoring: Actively monitor your sending reputation using tools like Google Postmaster Tools to identify and address potential issues.
- IP Change: In extreme cases, changing IP addresses might be necessary if the current IP is irrevocably tainted.
Marketer view
Marketer from Email Geeks mentions all domains have a *.mail.dk as their MX which belongs to Yousee - a broadband ISP.
17 Mar 2024 - Email Geeks
Marketer view
Email marketer from Reddit suggests checking if your IP address or the IP range it belongs to is listed on any public blacklists. Use online tools to check against various RBLs and delist if possible. Also, contact the recipient's email administrator to request whitelisting.
7 Mar 2022 - Reddit
What the experts say
3 expert opinions
The bounce message IP_IN_CIDR signifies that the sending IP address belongs to a CIDR (Classless Inter-Domain Routing) block which has been blacklisted. This is because the receiving mail server has detected a history of spam or malicious activity originating from that IP range. Consequently, even if the specific IP is not actively sending spam, it is blocked due to the overall negative reputation of its CIDR block.
Key opinions
- Blacklist Cause: IP_IN_CIDR indicates an IP is within a blocked CIDR range due to a local blacklist triggered by a history of spam.
- CIDR Blocking: The recipient mail server blocks an entire CIDR block based on historical spam activity, affecting all IPs within that range.
Key considerations
- Identify Blacklist: The first step is identifying the specific blacklist causing the IP_IN_CIDR error.
- Contact Operator: Contact the blacklist operator and provide evidence of efforts to prevent spam, which may lead to delisting.
- Authentication: Ensure proper configuration of email authentication methods like SPF, DKIM, and DMARC to improve email deliverability and prevent future blacklisting.
Expert view
Expert from Email Geeks suggests that IP_IN_CIDR indicates the IP is in a CIDR range and blocked due to a local blacklist.
18 Sep 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that IP_IN_CIDR means the recipient mail server has blocked a range of IP addresses (a CIDR block) due to a history of spam originating from that range. Even if a specific IP in that range isn't sending spam at the moment, it's still blocked because of the overall reputation of the block.
17 Sep 2023 - Spam Resource
What the documentation says
5 technical articles
The bounce message IP_IN_CIDR indicates that the sending IP address falls within a CIDR block that has been blacklisted by the recipient's mail server. This blacklisting typically occurs due to the CIDR range being associated with spam, policy violations, or other unwanted traffic. The receiving server rejects connections from any IP within that blocked CIDR range to protect its users from potentially harmful emails. The Spamhaus Block List (SBL) can indirectly influence IP_IN_CIDR errors, as a sending IP within a CIDR range blocked by Spamhaus may trigger the IP_IN_CIDR error.
Key findings
- Cause: IP_IN_CIDR errors arise when an IP belongs to a CIDR block blacklisted for spam or policy violations.
- Rejection: Receiving servers reject connections from any IP within a blacklisted CIDR range.
- Protection: This measure protects users from spam and unwanted emails originating from known problematic IP ranges.
- SBL Influence: The Spamhaus Block List (SBL) can indirectly influence IP_IN_CIDR errors.
Key considerations
- Address Blacklisting: Determine if your IP address is part of a blacklisted CIDR range.
- Review Policies: Understand and address any policy violations that may have led to blacklisting.
- Contact Blacklist: Contact the blacklist provider to understand the delisting process and requirements.
Technical article
Documentation from MailEnable explains the error means the client's IP address falls within a Classless Inter-Domain Routing (CIDR) range that has been specifically blocked by the recipient's mail server. This is usually because the IP range has been associated with spam or malicious activity.
6 Aug 2022 - MailEnable.com
Technical article
Documentation from Postfix clarifies that an IP_IN_CIDR error typically means the receiving mail server has determined the sending IP address belongs to a CIDR block associated with known spam sources or policy violations, leading to the rejection of the connection attempt.
10 Mar 2024 - Postfix.org
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