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

Marketer from Email Geeks mentions all domains have a *.mail.dk as their MX which belongs to Yousee - a broadband ISP.

Email marketer from Reddit suggests checking if your IP address or the IP range it belongs to is listed on any public blacklists. Use online tools to check against various RBLs and delist if possible. Also, contact the recipient's email administrator to request whitelisting.

Email marketer from Reddit advises to ensure your sending practices adhere to email best practices, including proper authentication (SPF, DKIM, DMARC), maintaining a clean email list, and avoiding spam triggers. This will help prevent your IP from being blacklisted.

Email marketer from Web Hosting Talk shares if you are using a cloud provider ensure their email infrastructure is well maintained. Some cloud providers have poor IP ranges and this can be a problem.

Email marketer from Stack Overflow shares that it suggests the sending mail server's IP address is within a CIDR block listed on a local blacklist. It's often due to spam activity originating from that IP range, causing the receiving server to block the entire range.

Email marketer from Quora states to actively monitor your sending reputation using tools like Google Postmaster Tools. This helps you identify and address any issues that could lead to your IP being blacklisted.

Email marketer from Web Hosting Talk suggests changing IP addresses, though this might be needed in extreme circumstances. If the IP address is irrevocably tainted, try a new one.

Email marketer from Super User recommends using tools like MXToolbox or MultiRBL to check if your IP or CIDR range is on any blocklists. These tools can provide insights into why you might be getting this error.

Email marketer from Email Marketing Forum shares that this error often occurs when a large block of IP addresses has been flagged for sending spam. If your sending server is part of that block, even if you aren't sending spam, your emails will be rejected.

Expert from Email Geeks suggests that IP_IN_CIDR indicates the IP is in a CIDR range and blocked due to a local blacklist.

Expert from Spam Resource explains that IP_IN_CIDR means the recipient mail server has blocked a range of IP addresses (a CIDR block) due to a history of spam originating from that range. Even if a specific IP in that range isn't sending spam at the moment, it's still blocked because of the overall reputation of the block.

Expert from Word to the Wise explains that to resolve IP_IN_CIDR issues, the first step is to identify the blacklist causing the problem. Contacting the blacklist operator and demonstrating that you're taking steps to prevent spam can lead to delisting. Also, ensure your email authentication (SPF, DKIM, DMARC) is properly configured.

Documentation from MailEnable explains the error means the client's IP address falls within a Classless Inter-Domain Routing (CIDR) range that has been specifically blocked by the recipient's mail server. This is usually because the IP range has been associated with spam or malicious activity.

Documentation from Postfix clarifies that an IP_IN_CIDR error typically means the receiving mail server has determined the sending IP address belongs to a CIDR block associated with known spam sources or policy violations, leading to the rejection of the connection attempt.

Documentation from Microsoft states the error indicates that the recipient server is configured to reject connections from IP addresses or IP ranges (CIDR blocks) known to be sources of spam or other unwanted traffic. This is done to protect the recipient's users from potentially harmful emails.

Documentation from Spamhaus explains that whilst not a direct answer the Spamhaus Block List (SBL) may affect the IP_IN_CIDR because if a sending IP is within a CIDR range that Spamhaus has blocked, it could result in the IP_IN_CIDR error message from a receiving mail server.

Documentation from Cisco explains that IP_IN_CIDR typically indicates that the sending IP address is part of a CIDR block that has been blacklisted due to spam or other policy violations. The receiving server is configured to reject connections from any IP within that CIDR range.