Why are emails to .dk domains being rejected with IP_IN_CIDR error?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jun 2025
Updated 16 Aug 2025
8 min read
It can be incredibly frustrating when your emails are rejected, especially when the bounce message, such as "IP_IN_CIDR," seems to defy a straightforward diagnosis. You check your server's PTR record, everything looks correct, and yet, emails destined for specific domains, like those ending in .dk, are consistently bounced. This is a scenario I've encountered that often leaves senders scratching their heads, as typical troubleshooting steps don't always reveal the root cause.
The "IP_IN_CIDR" error indicates that your sending IP address falls within a Classless Inter-Domain Routing (CIDR) block that the recipient's mail server has chosen to reject. While this might immediately make you think of public email blocklists (or blacklists), the reality is often more nuanced, particularly when dealing with specific country-code top-level domains.
My aim here is to shed light on why you might be seeing this particular rejection from .dk domains, despite what seems like a clean sending setup. We'll explore the less obvious causes behind this error and outline the steps you can take to diagnose and resolve these deliverability issues.
Understanding the IP_IN_CIDR error
The "IP_IN_CIDR" error message, often accompanied by a 554 5.7.1 status code, fundamentally means that the receiving mail server has a policy in place to reject emails from a specific range of IP addresses your sending server belongs to. This isn't necessarily due to your IP being listed on a major, well-known public blocklist (or blacklist). It can often stem from a private blocklist (or blacklist) maintained by the recipient's email provider, or a very specific network-level rule.
Many email providers maintain their own internal blocklists (also known as blacklists) based on their observed traffic patterns and spam detection algorithms. If a range of IPs, or even a single IP, exhibits suspicious behavior over time, it might be added to one of these private blocklists. Unlike public lists, these private lists are not searchable by external parties, which makes diagnosing the problem more challenging. For more information on what happens when your IP gets blocklisted, refer to our guide on what happens when your IP gets blocklisted.
Another common misconception is that a correct PTR (Pointer) record will prevent this error. While a properly configured PTR record is crucial for email deliverability and often a prerequisite for a receiving server to even consider your email, the presence of one does not override a direct IP or CIDR block. The bounce message itself often includes both the PTR and the IP, as seen in the example: 554 5.7.1 <proper.hostname.for.ip[1.1.1.1]>: Client host rejected: IP_IN_CIDR. This confirms that the PTR is being evaluated, but the block is happening at the IP level, regardless.
Private blocklists: The recipient's email provider may have added your IP or a range it belongs to their own internal blocklist (blacklist) due to past issues, even if it's not on public lists.
Geo-fencing: Some organizations block incoming connections from specific geographical regions or IP ranges known for spam activity, which your server might accidentally fall into.
Network reputation: If your IP is part of a larger network (CIDR block) that has a poor reputation, the entire block might be subject to stricter filtering rules, even if your specific IP is clean. You can learn more about general IP in CIDR errors.
Specific challenges with .dk domains
When you encounter an "IP_IN_CIDR" error specifically with .dk domains, it suggests that Danish email providers might have particular filtering mechanisms or policies in place. These can be more stringent or unique compared to general international email standards. For example, some regions or ISPs might have heightened security measures against spam or phishing attempts originating from specific IP ranges they deem suspicious.
Historical data often reveals that certain ISPs implement new, stricter rules or encounter specific issues that lead to widespread rejections. There have been reports, for instance, of users experiencing delivery issues to YouSee.dk emails with the exact same "IP_IN_CIDR" error. This suggests a localized issue or a specific policy change within certain Danish email ecosystems. Delving into local forums or news might sometimes reveal these specific circumstances, as one Danish forum discussion indicates about problems since April 2021, as seen on a Computerworld forum thread.
It's not uncommon for ISPs to implement geo-fencing or other advanced filtering based on the geographic location of the sending IP. While this isn't always explicitly stated as "geo-fencing" in the bounce, an "IP_IN_CIDR" error could imply that your IP's region or the network it belongs to is blocked due to security concerns or a history of spam originating from that area. It's crucial to consider these regional specificities when troubleshooting these types of rejections.
General IP_IN_CIDR issue
This typically occurs when a receiving server flags a sender's IP address (or its CIDR block) for suspicious activity, often due to a poor reputation or being on a private blocklist. It applies broadly across various recipient domains.
Scope: Affects multiple mailbox providers if the IP's reputation is poor or it's on a widely used blocklist (or blacklist). Our guide to email blocklists provides more context.
Resolution: Often involves reviewing sending practices, checking public blocklists (or blacklists), and improving overall sender reputation.
.dk domain specific issue
This points to stricter, localized policies or private blocklists (or blacklists) specific to Danish ISPs or domains. It suggests a targeted block by the receiving server based on geographic or internal criteria.
Scope: Primarily affects email delivery to Danish domains, even if other domains are receiving mail without issues.
Resolution: Requires direct communication with the postmaster of the specific .dk domain and investigation into unique regional policies.
Investigating and troubleshooting
When facing an "IP_IN_CIDR" rejection from .dk domains, the first crucial step is to directly contact the postmaster or support team of the problematic receiving domain. Since public blocklists (or blacklists) often don't show your IP, it's highly likely that the block is internal to their system. Providing them with the full bounce message, including the timestamp and your sending IP, can help them quickly identify the rule triggering the rejection and potentially whitelist your IP address.
Tips for contacting the postmaster
Be clear and concise: State the problem directly, including the exact error message and full bounce details. Avoid technical jargon where possible.
Provide relevant context: Explain your email sending practices, such as whether your emails are transactional, marketing, or personal, and how frequently you send. This helps illustrate your legitimate sending practices.
Offer to cooperate: Express willingness to adjust your sending practices if there's an issue on your end. This shows a commitment to good email hygiene.
Beyond contacting the postmaster, it's worth re-examining your Sender Policy Framework (SPF) record. While less common, I've seen instances where an SPF record with a ~all (SoftFail) mechanism, instead of a stricter -all (HardFail), can lead to unexpected rejections from very strict mail servers. Some older or highly cautious mail systems might interpret ~all as an indication of a less secure setup, even if it's technically compliant with SPF specifications. This is particularly relevant for demystifying SPF TempError issues.
Here's an example of an SPF record using -all which explicitly states that only the listed IPs are authorized to send email for your domain. Implementing a -all policy can sometimes resolve these stubborn rejections, especially from highly cautious recipients.
While directly addressing the IP_IN_CIDR error for .dk domains requires specific action, a proactive approach to email deliverability is always the best defense. This includes maintaining strong email authentication standards like SPF, DKIM, and DMARC. These records prove to receiving servers that your emails are legitimate and haven't been tampered with. You can find a simple guide to DMARC, SPF, and DKIM here.
Regularly monitoring your email deliverability is also critical. This means paying close attention to your bounce rates, specifically looking for patterns in error messages or recipient domains. If you notice a sudden spike in rejections from a particular ISP, it's a strong indicator that a new issue, similar to the .dk domain problem, might be emerging. Continuous monitoring helps you respond quickly to maintain optimal deliverability.
Ultimately, a robust email program built on trusted infrastructure and adherence to best practices will significantly reduce the likelihood of encountering such specific blocks. This includes ensuring your IP is not on any public blocklists (or blacklists), managing your sending volume responsibly, and maintaining clean recipient lists. These combined efforts contribute to a strong sender reputation, which is paramount for consistent inbox placement. Learn more about boosting email deliverability rates.
SPF mechanism
Description
Impact on deliverability
+all
Allows all sending IPs. Explicitly states that any IP can send mail for your domain.
Rarely used, as it essentially disables SPF protection and can lead to severe deliverability issues and being seen as spam.
?all
Neutral. Explicitly states no opinion on whether the IP is authorized.
Provides no real protection and minimal positive impact on deliverability. Generally discouraged.
~all
SoftFail. Allows unauthorized IPs but marks them as suspicious. It indicates that mail from IPs not listed in SPF should probably be treated with caution.
Commonly used during DMARC implementation. Can sometimes lead to emails landing in spam or being rejected by very strict receivers.
-all
HardFail. Explicitly states that any IP not listed in the SPF record is unauthorized and should be rejected.
Provides the strongest SPF protection. It's recommended for domains with stable sending infrastructure, as it clearly defines legitimate senders. Learn more about transitioning your DMARC policy.
Views from the trenches
Best practices
Proactively establish contact with specific domain postmasters for whitelisting.
Maintain perfectly configured SPF, DKIM, and DMARC records to build trust.
Common pitfalls
Assuming public blocklist (blacklist) status is the only cause for IP_IN_CIDR errors.
Neglecting to contact the recipient's postmaster directly for private blocks.
Expert tips
If you suspect geo-fencing, consider using an ESP with IP addresses located in the target region.
Always include the full bounce message when contacting postmasters; it provides crucial debugging information.
Marketer view
Marketer from Email Geeks says a block on the IP or network by the recipient mailbox provider is a likely cause. They advised checking if the IP is listed anywhere, reading other bounce messages for indicators, and contacting the postmaster if no public listings are found.
2021-05-19 - Email Geeks
Marketer view
Marketer from Email Geeks says if the recipient is a corporate email, requesting an IP whitelist from the receiving server might resolve the issue.
2021-05-19 - Email Geeks
Navigating stubborn deliverability challenges
Dealing with "IP_IN_CIDR" errors, especially from specific domains like those in Denmark, can be a complex challenge that goes beyond typical public blocklist (or blacklist) issues. It highlights the importance of understanding the nuances of how different email providers enforce their policies and the critical role of private blocklists (or blacklists).
By combining direct communication with the recipient's postmaster, a careful review of your SPF record, and consistent adherence to broader email deliverability best practices, you can navigate these challenges. Proactive monitoring of your sending reputation and swift action when bounce messages indicate new issues will ensure your emails continue to reach their intended recipients, regardless of the domain.
