Why am I getting IP in CIDR errors when sending emails?

Key findings

• Meaning of error: The 'IP in CIDR' bounce means your sending IP address is not within the acceptable range (CIDR block) as defined or expected by the recipient's mail server.
• Reverse DNS link: A common cause for this error is a misconfiguration or absence of proper reverse DNS (rDNS) for your sending IP. This prevents the recipient server from verifying the sender's identity.
• Recipient server changes: Errors can suddenly appear even without changes on your end, suggesting the receiving mail server (e.g., Mail.dk) has implemented new, stricter IP validation rules or policies.
• Conditional blocking: Sometimes, the blocking is conditional, affecting only specific IP ranges or certain sending patterns, indicating the recipient ISP might be selectively targeting certain sender behaviors.

Key considerations

• Verify rDNS: Ensure that your sending IP addresses have valid reverse DNS entries that correctly resolve to your domain. This is a fundamental email deliverability requirement, as explained by resources like DNS Made Easy.
• Monitor bounce messages: Analyze your bounce logs to understand the exact stage of the SMTP transaction where the rejection occurs, which can provide vital clues to the underlying cause. For more context, see What does the bounce message IP_IN_CIDR mean.
• Review sender practices: If only a portion of your mail or specific IPs are affected, investigate the content and address collection methods of those senders, as ISPs may be applying selective blocking due to perceived issues. You can also review how to troubleshoot 'connection refused' errors.

Key opinions

• Sudden appearance: Many marketers observe that these errors appear abruptly, affecting multiple senders simultaneously, even without recent DNS or infrastructure changes on their part.
• Partial impact: The issue frequently affects only a small percentage of mail for each sender, suggesting a conditional or targeted blocking rule rather than a complete block.
• Postmaster unresponsiveness: Some marketers find that contacting the recipient domain's postmaster email address yields little to no immediate resolution or helpful information.

Key considerations

• Systematic monitoring: Marketers should implement robust monitoring to detect sudden shifts in bounce rates, particularly for specific domains or error types like IP_IN_CIDR. This can help identify issues similar to those causing Microsoft 550 5.7.515 access denied bounces.
• Detailed logging: It is crucial to have systems that log the exact stage of the SMTP transaction where rejections occur, which helps in diagnosing conditional blocks and understanding why SPF might be failing for certain IPs.
• Internal review: If partial blocking occurs, examine the content and acquisition practices of the affected campaigns or senders, as the issue might stem from perceived spammy behavior, which can also contribute to emails going to spam.

Key opinions

• rDNS is key: Experts widely agree that 'IP in CIDR' errors are most likely a direct result of improper reverse DNS (rDNS) setup for the sending IP address.
• New recipient rules: The sudden appearance of these errors often indicates that the receiving ISP has recently implemented new, stricter, or conditional mail-stopping rules.
• Conditional application: The rule causing the error might be applied selectively, based on factors happening within the SMTP transaction itself, rather than a blanket block.
• Sender behavior red flag: If an ISP is selectively blocking certain sender IPs or patterns, it serves as a significant warning sign that the sender's mail practices are perceived as problematic, warranting a thorough internal audit.

Key considerations

• Transaction logging: Thoroughly review logs to determine at what specific point in the SMTP transaction the rejection occurs (e.g., after EHLO, after DATA). This can reveal the nature of the conditional rule, as highlighted by RIPE Labs on email sending over IPv6.
• Analyze differences: Compare successful and unsuccessful deliveries to identify any distinctions in content, addressing, or sending patterns that might trigger the blocking rule. This approach is similar to troubleshooting why an IP address might be blacklisted.
• Proactive reputation management: Beyond technical fixes, it is crucial to investigate the sender's content and address collection processes to understand why they might be targeted by spam filters. Understanding RBLs can also provide insights.

Key findings

• Reverse DNS necessity: Official documentation often states that a sending IP address should always have a valid reverse DNS entry that resolves correctly to its associated domain.
• Authentication requirements: Robust email authentication protocols, including SPF, DKIM, and DMARC, are crucial for validating the legitimacy of sending IPs and domains, reducing the likelihood of rejections.
• CIDR interpretation: CIDR (Classless Inter-Domain Routing) notation is used to define IP address ranges, and an 'IP_IN_CIDR' error means the sending IP falls outside the recipient's expected or allowed network block.
• Policy implementation: Mail servers can implement policies to reject connections or mail from IPs that do not meet specific criteria, such as proper rDNS or inclusion in an approved CIDR range.

Key considerations

• Adhere to standards: Ensure all email infrastructure adheres to established internet standards for IP addressing and DNS configuration, as this underpins reliable email delivery.
• Validate SPF records: Regularly check your SPF records to ensure they accurately list all authorized sending IP addresses and ranges, preventing issues where IPs are incorrectly excluded or included, as advised in the DOs and DON'Ts of using SPF records.
• Monitor network alignment: Confirm that your sending IP addresses are consistently aligned with your network's specified IP ranges to prevent mismatches that can trigger CIDR errors, and review the full form of SPF in email.
• Understand SMTP transaction states: Familiarize yourself with the various stages of the SMTP transaction, as rejection messages often occur at specific points, indicating the nature of the policy violation. This includes understanding the basics of DMARC, SPF, and DKIM.