Summary
The IP_IN_CIDR error when sending emails to .dk domains primarily indicates that the recipient mail server is blocking a range of IP addresses, including the sender's. This is often due to the IP address having a poor reputation stemming from spam activity, being listed on blocklists, or aggressive filtering by the recipient's mail server. Incorrectly configured SPF, DKIM, and DMARC records also contribute to deliverability issues. To resolve this, it's recommended to check the IP's reputation, ensure proper email authentication setup, contact the recipient's postmaster, verify reverse DNS settings, secure the sending infrastructure, and practice good list hygiene.
Key findings
- IP Blocklisting: The primary cause is the IP address falling within a blocked CIDR range on the recipient mail server.
- Poor Reputation: The sending IP address likely has a poor reputation due to spam activity, leading to blocking.
- Authentication Issues: Incorrectly configured SPF, DKIM, or DMARC records increase the likelihood of being blocked.
- Geo-Fencing: Some .dk domains may use geo-fencing or country-specific blocklists.
- Compromised Infrastructure: The sending infrastructure might be compromised and used for spam.
- rDNS Problems: Missing or mismatched reverse DNS (rDNS) records can lead to rejections.
Key considerations
- Contact Postmaster: Reach out to the recipient's mail server administrator or postmaster to request whitelisting or understand the blocking reason.
- Check Blocklists: Verify if the sending IP address is listed on public or private blocklists.
- Review Authentication: Thoroughly review and correct SPF, DKIM, and DMARC records to ensure proper authentication.
- Secure Infrastructure: Implement robust security measures to prevent the sending infrastructure from being compromised and used for spam.
- Monitor Reputation: Continuously monitor IP address reputation and take proactive steps to maintain a good sender reputation.
- Implement List Hygiene: Proactive management to minimise spam traps and list bomb attacks.
- Reverse DNS: Ensure a correct reverse DNS.
What email marketers say
12 marketer opinions
Emails to .dk domains being rejected with the IP_IN_CIDR error often stem from the recipient mail server blocking the sender's IP range due to perceived spam risks. Several factors can contribute to this, including poor IP reputation, incorrect email authentication setup (SPF, DKIM, DMARC), presence on blocklists, geo-fencing policies, or even a compromised sending infrastructure. Troubleshooting involves checking the IP's reputation, verifying authentication records, contacting the recipient's postmaster for whitelisting, and ensuring the sending infrastructure is secure and properly configured.
Key opinions
- IP Reputation: The sending IP address likely has a poor reputation, causing the .dk domains to block the IP range.
- Email Authentication: Incorrectly configured SPF, DKIM, or DMARC records can lead to deliverability issues, including IP_IN_CIDR errors. An SPF record using ~all instead of -all may also be a cause.
- Blocklists: The sending IP might be listed on public or private blocklists used by .dk domains.
- Geo-Fencing: Some .dk domains might use geo-fencing or country-specific blocklists, blocking IPs from certain regions.
- Compromised Infrastructure: The sending infrastructure may be compromised and used for spam, leading to IP blocking.
- rDNS Mismatch: A reverse DNS mismatch or lack of configuration can cause security filters to cause rejections.
Key considerations
- Contact Postmaster: Reach out to the postmaster of the .dk domain to request whitelisting or understand the specific reason for the block.
- Monitor Reputation: Use reputation monitoring tools to check the IP's reputation and take steps to improve it.
- Verify Authentication: Ensure SPF, DKIM, and DMARC records are correctly configured and that the sending server's IP address is included in the SPF record.
- Check for Malware: Thoroughly check the sending servers for malware and ensure that security best practices are followed.
- Review Security Policies: Recipient mail servers have stricter security policies in place so whitelisting might be necessary to avoid rejections.
Marketer view
Email marketer from Reddit shares their experience encountering this issue, noting that it's often difficult to resolve directly. They suggest contacting the postmaster of the .dk domain and asking for specific reasons or whitelist options. Also suggest checking if the IP used to send emails is on any public or private blocklists used by the .dk domains.
24 Nov 2024 - Reddit
Marketer view
Email marketer from Email Geeks suggests that an "incorrect" SPF record could be the cause, potentially a ~all instead of -all at the end of the SPF record. Also points out that there are issues with YouSee.dk emails, as many people are having the same issue since 1.4.2021.
4 Nov 2024 - Email Geeks
What the experts say
3 expert opinions
Emails to .dk domains being rejected with the IP_IN_CIDR error indicate that the recipient mail server has blocked the sender's IP range. This blockage is often due to factors such as poor IP reputation, spam activity originating from the IP range, aggressive filtering by the recipient server, or being listed on blocklists. Addressing this involves investigating the IP's reputation, confirming proper email authentication (SPF, DKIM, DMARC), contacting the recipient's mail server, and implementing proactive list hygiene and monitoring processes.
Key opinions
- IP Blocking: The recipient mail server has blocked the sender's IP range due to perceived spam risks.
- Poor IP Reputation: A poor IP reputation stemming from spam activity or other negative factors can lead to blocking.
- Aggressive Filtering: Recipient mail servers may employ aggressive filtering rules, resulting in IP_IN_CIDR errors.
- Blocklisting: The IP address may be listed on public or private blocklists.
- Poor List Hygiene: Spam traps, complaints, bounces, and poor list hygiene contribute to the IP's reputation and potential blocklisting.
Key considerations
- Check Reputation: Investigate the IP address's reputation on public blocklists and through reputation monitoring services.
- Verify Authentication: Ensure SPF, DKIM, and DMARC records are correctly configured to authenticate emails.
- Contact Postmaster: Contact the recipient's mail server administrator to request whitelisting or further clarification on the blocking reason.
- Proactive Monitoring: Implement a proactive monitoring process to maintain list hygiene and avoid spam traps, complaints, and bounces.
Expert view
Expert from Email Geeks explains that the issue sounds like a block on the IP or the network by the recipient mailbox provider. Suggests checking if the IP is listed anywhere and addressing it. Also recommends reading bounces from other MBPs for blocking indicators and contacting postmasters for tips.
4 Apr 2025 - Email Geeks
Expert view
Expert from SpamResource explains that IP_IN_CIDR errors typically indicate that the recipient mail server has blocked a range of IP addresses that includes the sender's IP. This can be due to poor IP reputation, spam activity originating from the IP range, or simply overly aggressive filtering. They recommend checking the IP against public blocklists, ensuring proper SPF/DKIM/DMARC configuration, and contacting the recipient's mail server administrator to request whitelisting or clarification.
22 Jan 2023 - SpamResource
What the documentation says
5 technical articles
The IP_IN_CIDR error when sending emails to .dk domains signifies that the recipient server's anti-spam policy is rejecting connections from the sender's IP address, often because it falls within a blocked CIDR range associated with spam activities. Email authentication issues, specifically with SPF, DKIM, and DMARC, can also negatively impact deliverability and increase the likelihood of being blocked. Proper configuration and adherence to relevant RFCs are crucial for email authentication to ensure deliverability.
Key findings
- CIDR Block Listing: .dk domains reject connections from IP addresses within certain CIDR blocks due to anti-spam policies.
- Spamhaus Blocklists: IP addresses listed in Spamhaus blocklists are rejected by servers utilizing those lists.
- DMARC Failures: DMARC failures can negatively affect email deliverability and increase the risk of being blocked.
- SPF Record Syntax: Improper SPF record syntax can lead to deliverability issues.
- Invalid DKIM Signatures: Emails lacking valid DKIM signatures are more likely to be flagged as spam or rejected.
Key considerations
- Check Blocklists: Check if the sending IP address is listed on public blocklists like Spamhaus.
- Configure Authentication: Ensure SPF, DKIM, and DMARC are properly configured to authenticate emails.
- Validate SPF Syntax: Adhere to RFC 7208 for proper SPF record syntax.
- Ensure Valid DKIM: Maintain consistent and valid DKIM signatures for email authentication.
- Monitor Reputation: Monitor IP and domain reputation to proactively address deliverability issues.
Technical article
Documentation from RFC Editor (RFC 7208) explains proper syntax and semantics for SPF records. Following the RFC will ensure the record is correctly interpereted, and can reduce the risk of errors that might lead to deliverability issues.
28 Feb 2023 - RFC Editor
Technical article
Documentation from DKIM.org describes the DKIM signing process, highlighting the importance of consistent and valid DKIM signatures for email authentication and deliverability. It emphasises that emails lacking valid DKIM signatures are more likely to be flagged as spam or rejected.
8 Aug 2023 - DKIM.org
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