Why are emails to .dk domains being rejected with IP_IN_CIDR error?

Email marketer from Reddit shares their experience encountering this issue, noting that it's often difficult to resolve directly. They suggest contacting the postmaster of the .dk domain and asking for specific reasons or whitelist options. Also suggest checking if the IP used to send emails is on any public or private blocklists used by the .dk domains.

Email marketer from Email Geeks suggests that an "incorrect" SPF record could be the cause, potentially a ~all instead of -all at the end of the SPF record. Also points out that there are issues with YouSee.dk emails, as many people are having the same issue since 1.4.2021.

Email marketer from Mailjet Support says if the SPF and DKIM records are correct, but you are still getting this message, they recommend reaching out to the recipient or their email provider to request whitelisting or to understand the specific reason for the block. They explain it's possible that the recipient's mail server has stricter security policies in place.

Email marketer from Stack Overflow states that this error could be due to an SPF record that is either missing or misconfigured. The user explains that it is important to ensure the sending server's IP address is included in the SPF record for the sending domain.

Email marketer from Postmark Support says to ensure that reverse DNS (rDNS) is properly configured for your sending IP address. The rDNS record should match the hostname used in your HELO/EHLO greeting. Mismatched or missing rDNS can trigger security filters and lead to rejections.

Email marketer from Email Geeks recommends reaching out to the postmaster or the proper channel to resolve the issue, sharing an anecdote about accidentally blocking someone due to geo-fencing.

Expert from Email Geeks explains that the issue sounds like a block on the IP or the network by the recipient mailbox provider. Suggests checking if the IP is listed anywhere and addressing it. Also recommends reading bounces from other MBPs for blocking indicators and contacting postmasters for tips.

Email marketer from EmailDiscussions Forum mentions that some .dk domains might use geo-fencing or country-specific blocklists. They suggest checking if the sending IP address is located in a country that is blocked by the recipient's mail server.

Email marketer from Email Geeks suggests that if the recipient is using a corporate email, requesting an IP whitelist from the receiving server might resolve the issue.

Email marketer from Quora suggests that the client's sending infrastructure could be compromised and used for spam. They recommend thoroughly checking the sending servers for malware and ensuring that security best practices are followed.

Email marketer from Email Geeks suggests that an "incorrect" SPF record could be the cause, potentially a ~all instead of -all at the end of the SPF record. Also points out that there are issues with YouSee.dk emails, as many people are having the same issue since 1.4.2021.

Email marketer from EmailSecurityPro Forum suggests the IP address might have a poor reputation score. They advise using reputation monitoring tools to check the IP's reputation and taking steps to improve it, such as ensuring proper email authentication (SPF, DKIM, DMARC) and avoiding spam triggers in email content.

Expert from Email Geeks explains that the issue sounds like a block on the IP or the network by the recipient mailbox provider. Suggests checking if the IP is listed anywhere and addressing it. Also recommends reading bounces from other MBPs for blocking indicators and contacting postmasters for tips.

Expert from SpamResource explains that IP_IN_CIDR errors typically indicate that the recipient mail server has blocked a range of IP addresses that includes the sender's IP. This can be due to poor IP reputation, spam activity originating from the IP range, or simply overly aggressive filtering. They recommend checking the IP against public blocklists, ensuring proper SPF/DKIM/DMARC configuration, and contacting the recipient's mail server administrator to request whitelisting or clarification.

Expert Laura Atkins from Word to the Wise explains that blocklisting, which would result in an IP_IN_CIDR error, is complex and results from many factors. These factors include spam traps, complaints, bounces and poor list hygiene. A proactive monitoring process can help avoid these scenarios.

Documentation from RFC Editor (RFC 7208) explains proper syntax and semantics for SPF records. Following the RFC will ensure the record is correctly interpereted, and can reduce the risk of errors that might lead to deliverability issues.

Documentation from DKIM.org describes the DKIM signing process, highlighting the importance of consistent and valid DKIM signatures for email authentication and deliverability. It emphasises that emails lacking valid DKIM signatures are more likely to be flagged as spam or rejected.

Documentation from Spamhaus explains that CIDR block listing is a common practice to block large ranges of IP addresses that are associated with spam activities. If an IP falls within a listed CIDR block, emails from that IP will be rejected by servers that use the Spamhaus blocklists.

Documentation from Microsoft Learn explains that the IP_IN_CIDR error typically indicates that the receiving mail server (.dk domain in this case) has an anti-spam policy that rejects connections from IP addresses within certain CIDR blocks. This is often used to block entire ranges of IPs known for sending spam.

Documentation from DMARC.org explains that DMARC failures, even if not directly causing the IP_IN_CIDR error, can negatively impact email deliverability. Ensuring that DMARC is properly configured, along with SPF and DKIM, can improve your sending reputation and reduce the likelihood of being blocked.