What does the bounce message IP_IN_CIDR mean and what causes it?

Key findings

• Specific block: IP_IN_CIDR signifies that the blocking is based on your IP being part of a larger network range, not just an individual address.
• Local blacklist: This error frequently arises from internal or private blocklists (also known as blacklists) managed by the recipient's mail server or ISP. Unlike public blocklists, these are not usually searchable.
• Geographic concentration: The issue can be concentrated within specific regions or countries, affecting multiple seemingly unrelated domains that share common infrastructure or ISPs in that area, such as domains in Denmark.
• Required investigation: Resolving this bounce typically necessitates direct communication and investigation with the postmaster or abuse desk of the affected recipient domain's ISP.

Key considerations

• Deliverability impact: Being on such a localized blocklist, or blacklist, can severely hinder your ability to reach subscribers in the affected region, leading to significant deliverability issues.
• Proactive reputation management: Regular monitoring of your IP and domain reputation is crucial to identify and address issues that could lead to blocklisting on any type of blocklist, including private ones.
• Postmaster engagement: The most effective way to address IP_IN_CIDR errors is to contact the postmaster of the blocking domain to understand their specific policy and request delisting of your IP or CIDR range. More general information about common email bounce messages can be found on Rackspace Technology Documentation.
• Sending practices: Reviewing and optimizing your sending volume, frequency, and recipient engagement can help maintain a positive sender reputation and prevent future blocks, particularly if your IP falls on a blocklist.

Key opinions

• Uncommon error: Many marketers are unfamiliar with this specific bounce message, which causes confusion and delays in finding solutions.
• Suspected local blocklist: There's a common suspicion that this error stems from a private or local blacklist (also known as blocklist) when it affects multiple, unrelated domains within a specific country or region.
• Geographic pattern: The recurrence of these bounces, especially from domains in a particular country, strongly suggests a region-specific blocking policy or a local ISP's internal reputation system.
• Direct investigation: Marketers often conclude that direct investigation and communication with the affected ISPs are the only viable paths to resolution, as public search results are often unhelpful.

Key considerations

• Client impact: This bounce type can significantly impede a client's ability to engage with their audience in a specific geographical area, demanding immediate attention.
• Data collection: Compiling a comprehensive list of all affected domains, including any sub-domains, is essential to identify commonalities, such as shared MX records and underlying ISPs, to pinpoint the source of the block.
• ISP engagement: Establishing contact with the postmaster or abuse desk of the blocking ISP is often the most direct route to understanding the specific policy that led to the IP_IN_CIDR error and requesting delisting.
• Persistence: Resolving these less common bounce codes requires persistent effort, sometimes involving reaching out to multiple contacts within an organization or ISP.

Key opinions

• CIDR significance: Experts agree that IP_IN_CIDR definitively points to a block based on the sending IP being part of a larger network range, rather than an isolated issue with a single IP address.
• Private blocklists: The most common underlying cause for this bounce is private or internal blocklists (blacklists) that are managed by the recipient's ISP, distinguishing them from the more commonly known public blocklists.
• ISP-specific policy: Such blocks often reflect unique email receiving policies or specific thresholds implemented by individual ISPs to manage incoming mail flow and mitigate abuse.
• Root cause analysis: Resolving these issues requires a deeper analysis of the entire CIDR block's sending patterns and its overall reputation, as the block may not be tied to a single instance of abuse.

Key considerations

• Network reputation: The overall reputation of your entire IP range or CIDR block, not just one specific IP, can be a major factor in these types of blocks, making it important to monitor your IP reputation comprehensively.
• Localized troubleshooting: Be prepared for highly localized and specific troubleshooting efforts, as IP_IN_CIDR blocks are not typically global and vary by recipient.
• Postmaster communication: Direct, polite, and detailed communication with the target ISP's postmaster or abuse contact is often the only pathway to understanding the specific reason for the block and negotiating for delisting. For more insights on how blacklists actually work, consider exploring further resources.
• IP warm-up and hygiene: Maintaining consistent, good sending practices and properly warming up any new IP addresses are critical to preventing such broad blocks from occurring in the first place, safeguarding your overall deliverability.

Key findings

• Technical definition: IP_IN_CIDR indicates that the connecting IP address matches an entry within a predefined CIDR range that has been configured for blocking on the receiving mail server.
• Server configuration: This type of block is commonly implemented through mail server configuration rules, such as those found in Postfix, Exim, or Sendmail, which allow administrators to reject connections from specific IP ranges or entire network subnets.
• Abuse prevention: Documentation typically states that CIDR-based blocks serve as a broad measure to prevent abuse or unwanted traffic originating from known problematic networks or those associated with high volumes of spam. This is a common tactic, as detailed in an in-depth guide to email blocklists.
• Block duration: Depending on the specific policy of the blocking entity, such blocks can be temporary or permanent, and they are typically maintained either manually by administrators or automatically by sophisticated reputation systems.

Key considerations

• Network segmentation: ISPs and large organizations frequently use CIDR notation to effectively manage and segment their networks, making it a logical unit for applying email policy enforcement and controlling traffic.
• Dynamic vs. static lists: Some email security systems may dynamically add CIDR blocks to internal blacklists based on real-time threat intelligence and observed spamming behavior, while others might rely on more static, predefined configurations. Understanding what a DNSBL is can provide context.
• RFC compliance: While the specific IP_IN_CIDR bounce message is proprietary, the underlying practice of blocking email based on IP addresses is a standard and accepted practice in email security, even if not explicitly detailed in all RFCs for every error message.
• Postmaster role: Documentation often implicitly or explicitly advises senders to contact the recipient's postmaster or abuse desk if they believe their IP address or CIDR block has been mistakenly or unfairly blocked, as these blocks are locally managed.