Summary
The consensus among various experts, marketers, and documentation sources indicates that 'IP in CIDR' errors when sending emails are primarily caused by issues related to reverse DNS (rDNS) configurations, particularly missing or incorrect PTR records. These records are crucial for verifying the sending server's identity by mapping its IP address back to its hostname. Further factors contributing to this issue include poor IP reputation, SMTP banner mismatches, using dynamic IP addresses, and inconsistencies in forward and reverse DNS records. Diagnosing the specific point of rejection within the SMTP transaction can also provide valuable insights. Contacting your ISP or hosting provider to configure the PTR record correctly, as well as using diagnostic tools to assess DNS configuration and IP reputation, are frequently recommended steps.
Key findings
- PTR Record Issues: Missing or incorrect PTR records are the most prevalent cause of 'IP in CIDR' errors.
- rDNS Importance: Proper reverse DNS setup, facilitated by accurate PTR records, is crucial for verifying sending server identity.
- IP Reputation Impact: A poor IP reputation can result in email rejections, with servers blocking IPs listed on blocklists.
- SMTP Banner Mismatch: Discrepancies between the SMTP banner and the PTR record can lead to rejections.
- Dynamic IP Problems: Using dynamic IP addresses, often blocked by ISPs to prevent spam, is another contributing factor.
- DNS Consistency: Ensuring consistency between forward and reverse DNS records is essential.
- Conditional Blocking: The error may be conditional, requiring analysis of the SMTP transaction.
Key considerations
- Validate PTR Record: Verify that the sending server's IP address has a valid PTR record resolving to the correct hostname.
- Monitor IP Reputation: Regularly check IP reputation against blocklists and take corrective action if necessary.
- Synchronize SMTP Banner: Ensure the SMTP banner matches the hostname specified in the PTR record.
- Contact ISP/Hosting Provider: Contact your ISP or hosting provider to configure or correct PTR records, as they control the reverse DNS zone.
- Use Static IP Address: Consider using a static IP address instead of a dynamic IP address.
- Analyse SMTP Transactions: Analyse your SMTP transactions, to see exactly at which point errors occur.
- Investigate sender practices: Investigate sender content and address collection practices
- Employ Diagnostic Tools: Utilize tools like MXToolbox to diagnose DNS configurations and identify potential problems.
What email marketers say
6 marketer opinions
The primary reason for 'IP in CIDR' errors when sending emails is related to issues with reverse DNS (PTR) records. These records, which map an IP address back to a domain name, are frequently missing or incorrectly configured, causing recipient mail servers to reject the connection as they cannot verify the sender's identity. Additionally, IP reputation, SMTP banner mismatches, and inconsistent forward/reverse DNS configurations can contribute to this issue. Ensuring your SMTP banner matches your hostname, checking if you are on any blocklists and contacting your ISP to fix the RDNS records are further suggested steps.
Key opinions
- PTR Record Issues: Missing or incorrect PTR records are the most common cause of 'IP in CIDR' errors. These records must be properly configured to match the sending server's hostname.
- IP Reputation: A poor IP reputation can lead to rejections, with receiving servers blocking IPs listed on blocklists.
- SMTP Banner Mismatch: If the SMTP banner (the hostname presented during the SMTP handshake) does not match the PTR record, receiving servers might reject the connection.
- DNS Consistency: Forward and reverse DNS records must be consistent. The sending server's hostname should resolve back to the IP address.
Key considerations
- Verify PTR Record: Check that your sending server's IP address has a valid PTR record that resolves to the correct hostname. Use online tools to verify the configuration.
- Monitor IP Reputation: Regularly check your IP address against blocklists to ensure it's not listed. Take steps to improve your IP reputation if necessary.
- Match SMTP Banner: Ensure that the hostname presented in your SMTP banner matches the hostname specified in your PTR record.
- Contact ISP: Contact your ISP or hosting provider to configure or correct your PTR records, as they control the reverse DNS zone for your IP address.
Marketer view
Email marketer from Reddit explains that IP in CIDR errors could stem from either missing or incorrect reverse DNS records, or a negative IP reputation. The user recommends confirming that the sending IP has a valid PTR record pointing back to the server's hostname and that the IP is not blacklisted.
13 Feb 2025 - Reddit
Marketer view
Email marketer from SparkPost suggests that IP reputation could be the cause, where a bad IP reputation is the reason the messages are getting blocked. Checking if the IP is on any blocklists.
4 Dec 2024 - SparkPost
What the experts say
4 expert opinions
The 'IP in CIDR' error typically arises from issues with reverse DNS (rDNS) configurations, particularly relating to PTR records. Setting up a proper PTR record is crucial for verifying the legitimacy of the sending server, as it ensures the IP address resolves back to the domain name. Using dynamic IP addresses, which are often blocked by ISPs to prevent spam, can also trigger this error. Additionally, the error may be conditional, necessitating an examination of the SMTP transaction to identify the specific point of rejection and differences between successful and unsuccessful deliveries. Investigating sender content and address collection practices is also important.
Key opinions
- Reverse DNS Setup: The primary cause is related to how your reverse DNS is set up.
- PTR Records Importance: PTR records are essential for proper reverse DNS, ensuring IP addresses resolve to domain names.
- Dynamic IPs: Using dynamic IP addresses is a common reason for rejections.
- Conditional Blocking: The error might be conditional, influenced by factors in the SMTP transaction.
Key considerations
- Verify rDNS: Ensure your reverse DNS is properly configured.
- Implement PTR Records: Set up proper PTR records to map IP addresses to domain names.
- Use Static IP: Consider using a static IP address instead of a dynamic one.
- SMTP Transaction Analysis: Investigate the SMTP transaction to determine the exact point of rejection.
- Sender Practices: Investigate sender content and address collection practices
Expert view
Expert from Word to the Wise explains the importance of PTR records. A proper PTR record ensures reverse DNS is set up correctly, where the IP address resolves back to the domain name used for sending emails. They emphasize that mail servers often use this to verify the legitimacy of the sending server. They also explain about reverse DNS not being delegated.
24 Jul 2021 - Word to the Wise
Expert view
Expert from Email Geeks explains that IP in CIDR likely refers to how your reverse DNS is set up.
22 Sep 2021 - Email Geeks
What the documentation says
3 technical articles
The primary cause of 'IP in CIDR' errors, according to documentation from Microsoft, cPanel, and MXToolbox, is related to reverse DNS (rDNS) lookup failures, particularly involving missing or incorrect PTR records. These records are essential for verifying the identity of the sending server by mapping its IP address back to its hostname. Proper DNS configuration is also emphasized, with the recommendation to contact your ISP or hosting provider to set up the PTR record, as they control the reverse DNS zone. Tools like MXToolbox can be used to diagnose DNS configurations.
Key findings
- Reverse DNS Failure: IP in CIDR errors often relate to reverse DNS lookup failures.
- Missing/Incorrect PTR Records: A missing or incorrect PTR record is the most likely cause of the error.
- Identity Verification: PTR records are used to verify the sending server's identity.
Key considerations
- Valid PTR Record: Ensure your sending server's IP address has a valid PTR record that resolves back to the server's hostname.
- Verify DNS Configuration: Verify the DNS configuration for any discrepancies.
- Contact ISP/Hosting Provider: Contact your ISP or hosting provider to set up the PTR record, as they control the reverse DNS zone for your IP address block.
- Use Diagnostic Tools: Use tools like MXToolbox to diagnose DNS configurations.
Technical article
Documentation from cPanel Documentation outlines how to configure reverse DNS for your server. It stresses the importance of contacting your ISP or hosting provider to set up the PTR record, as they control the reverse DNS zone for your IP address block. Also, the documentation highlights the importance of ensuring that the hostname resolves correctly to the IP address.
24 Nov 2021 - cPanel Documentation
Technical article
Documentation from MXToolbox describes that a missing or incorrect reverse DNS (PTR) record is the most likely cause of the error, as this is used to verify the sending server's identity. It is also explained that you can use MXToolbox to diagnose the DNS configuration.
4 Oct 2024 - MXToolbox
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