What does the bounce message IP_IN_CIDR mean and what causes it?
Michael Ko
Co-founder & CEO, Suped
Published 9 Jun 2025
Updated 16 Aug 2025
7 min read
When you send emails, you expect them to land in the recipient's inbox. However, sometimes your messages are returned with a bounce message, also known as a Non-Delivery Report or NDR, indicating that the delivery failed. These messages contain codes and descriptions that can help you diagnose the issue. One such specific bounce message you might encounter is 'IP_IN_CIDR'.
The 'IP_IN_CIDR' bounce message means the recipient's mail server has rejected your email because your sending IP address, or a broader range of IP addresses it belongs to (a CIDR block), is on a local blocklist (or blacklist) or is deemed untrustworthy by that server. Understanding this message is crucial for troubleshooting and maintaining your email deliverability.
Understanding the IP_IN_CIDR bounce message
554 5.7.1 status code, signals a rejection based on the IP address of the sending server. It indicates that the recipient's server (or an upstream filtering service) has identified your IP within a specifically configured block of addresses and has chosen to reject mail from that entire range. This typically suggests a localized blacklist (or blocklist) or reputation flagging.
CIDR, or Classless Inter-Domain Routing, is a method of allocating IP addresses and routing Internet Protocol packets more efficiently. Instead of assigning individual IPs, networks can designate a range of IPs using CIDR notation, such as 192.168.1.0/24. When a mail server sees an IP_IN_CIDR bounce, it means your IP fell into one of these defined ranges that the receiving server has decided to block for various reasons.
The appearance of 'IP_IN_CIDR' messages, particularly from specific top-level domains like .dk (Denmark), often points to regional or ISP-specific blocklists. This suggests that certain mail exchange (MX) hosts or broadband providers might implement their own blocking criteria based on the IP ranges they receive mail from. You might also encounter related issues, such as why emails to .dk domains are rejected with this error.
Causes of IP_IN_CIDR bounces
The primary cause of an 'IP_IN_CIDR' bounce is the presence of your sending IP, or its associated CIDR block, on a recipient's local blacklist (or blocklist). Unlike widely known public blacklists, these are private lists maintained by individual Internet Service Providers (ISPs), corporations, or even specific email administrators to protect their networks from perceived threats.
Your IP's reputation plays a significant role in this. If your sending IP has a poor reputation due to factors like high spam complaints, sending to inactive or invalid email addresses, or being associated with spam campaigns, an ISP might proactively block its entire CIDR range to prevent unwanted mail. This is a core reason why emails bounce, as discussed in detail on HubSpot's knowledge base on bounce types. To understand more about why emails bounce generally, you can explore additional insights.
Another common cause, especially for senders using Email Service Providers (ESPs) or shared hosting, involves shared IP pools. If other senders using IP addresses within the same CIDR block as yours engage in problematic sending behavior, it can negatively impact the reputation of the entire block. This collective reputation can lead to your emails being rejected. Geographic or regional specific blocklists can also contribute, as seen with the .dk domains, where some countries or ISPs might have unique filtering policies particularly sensitive to certain IP ranges.
Diagnosing and resolving IP_IN_CIDR issues
The first step in diagnosing an 'IP_IN_CIDR' bounce is a careful review of the Non-Delivery Report (NDR). The bounce message usually provides specific details, including the exact IP address that was rejected and the 'IP_IN_CIDR' error code. This immediate feedback is invaluable for understanding the specific failure point, which can be part of common email bounce messages and their meanings.
While 'IP_IN_CIDR' often implies a local blocklist (or blacklist), it's still prudent to check whether your IP is listed on any major public DNS-based blocklists (DNSBLs). A presence on these lists could escalate local blocks. You can use standard command-line tools to query DNSBLs. For example, to check an IP against the Zen Spamhaus blocklist (SBL), you might use a command similar to this:
Example DNSBL lookupBASH
dig +short 4.3.2.1.sbl.spamhaus.org
Long-term resolution hinges on reviewing and improving your overall sending practices. This includes verifying your sender authentication records like SPF, DKIM, and DMARC, regularly cleaning your email lists, and ensuring your content isn't triggering spam filters. Poor list hygiene, in particular, can lead to invalid user bounces beyond IP reputation. Understanding bounce message error codes can greatly aid in troubleshooting various deliverability challenges.
Preventing IP_IN_CIDR bounces
Maintaining a consistently good IP reputation is paramount to preventing 'IP_IN_CIDR' bounces and ensuring your emails reach their intended destination. This involves rigorous management of your email lists to remove inactive or problematic addresses, and a commitment to sending only relevant and solicited content. Properly configured authentication protocols are also vital.
For organizations sending high volumes of email, strategically segmenting your email streams can significantly mitigate risk. Separating transactional emails from marketing campaigns, and potentially utilizing dedicated IP addresses for each, helps isolate any reputation issues to a smaller scope. This means an issue with marketing emails won't inadvertently impact your critical transactional communications.
Proactive monitoring of your IP addresses across various blacklists (or blocklists) is a non-negotiable best practice. Regular checks can alert you to potential listings before they escalate into widespread delivery problems, including 'IP_IN_CIDR' errors. Utilizing a blocklist checker or setting up continuous Suped blocklist monitoring can help you respond swiftly to any emerging issues, safeguarding your sender reputation. For a comprehensive understanding of how email blacklists actually work, consider exploring a dedicated guide.
Tips for contacting postmasters
Gather evidence: Provide logs, bounce messages, and details of your sending practices.
Be polite and professional: Maintain a respectful tone in all communications.
Clearly state the problem: Explain the 'IP_IN_CIDR' bounce and the affected IP(s).
Outline corrective actions: Describe what steps you have taken to resolve the underlying issue.
Request clarification: Ask if they can provide more specific reasons for the block.
Views from the trenches
Best practices
Regularly audit your email lists to remove inactive or bouncing addresses, which improves overall list hygiene.
Implement strong email authentication protocols (SPF, DKIM, DMARC) to build and protect your sender reputation.
Monitor major and local blocklists for your sending IPs to catch and address listings promptly.
Common pitfalls
Ignoring initial 'IP_IN_CIDR' bounce messages, allowing issues to escalate and impact more recipients.
Using shared IP pools without adequate oversight, making you vulnerable to other senders' poor practices.
Failing to maintain a clean email list, which can lead to spam traps and increased bounce rates.
Expert tips
Contacting the postmaster of the blocking domain directly can often provide specific reasons for the block.
If you are seeing blocks from a specific regional ISP, consider if your content or sending patterns might be triggering their local filters.
For recurring issues with specific ISPs, consider exploring a dedicated IP address if shared pools continue to be a problem.
Expert view
Expert from Email Geeks says that the IP_IN_CIDR message typically indicates that the sender's IP falls within a CIDR range that has been locally blocklisted by the recipient's server.
April 7, 2021 - Email Geeks
Marketer view
Marketer from Email Geeks notes that many of the affected .dk domains share MX records pointing to mail.dk, suggesting a common broadband ISP (Yousee) as the potential source of the block.
April 7, 2021 - Email Geeks
Ensuring deliverability with IP_IN_CIDR challenges
The 'IP_IN_CIDR' bounce message, while seemingly cryptic, provides a clear signal that your email has been rejected because your IP address (or its associated block) is on a local or specific blocklist. This type of rejection underscores the nuanced nature of email deliverability, where even a single network's policy can impact your sending.
Proactive management, including vigilant reputation monitoring, robust authentication, and meticulous list hygiene, is your best defense against such bounces. By understanding what causes these specific rejections and how to address them, you can significantly improve your email deliverability and ensure your messages consistently reach their intended inboxes.
