Why am I getting IP in CIDR errors when sending emails?
Michael Ko
Co-founder & CEO, Suped
Published 29 Jul 2025
Updated 19 Aug 2025
9 min read
Email deliverability can sometimes feel like navigating a complex maze, especially when you encounter cryptic error messages. One such message that can halt your email campaigns is "IP in CIDR" (Classless Inter-Domain Routing). This error indicates that the receiving mail server is rejecting your email because your sending IP address, or the range it belongs to, isn't authorized or is violating a specific policy. It's a signal that something is amiss with how your IP is perceived or configured.
When you encounter an "IP in CIDR" error, it means the recipient's mail server has a defined list of allowed or blocked IP ranges, and your sending IP doesn't fit the expected pattern. This isn't just a generic rejection, it's a specific flag indicating a network configuration or policy mismatch. Understanding CIDR notation is key to resolving these issues.
Troubleshooting this kind of bounce requires a deep dive into your email authentication records, particularly SPF, and how recipients, like those at Mail.dk, validate incoming mail. It often points to a misalignment between your declared sending sources and the actual IP addresses used.
Understanding CIDR and email
CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses and routing Internet Protocol packets. It's a way to specify a range of IP addresses efficiently using a base IP address and a suffix (e.g., /24, /32) that indicates the number of bits in the network mask. For example, 192.168.1.0/24 defines a network with 256 possible IP addresses.
How CIDR works with email security
Mail servers use CIDR notation in various security mechanisms, most notably SPF (Sender Policy Framework) records. An SPF record is a DNS TXT record that lists all authorized IP addresses or domains permitted to send email on behalf of your domain. If an email arrives from an IP address not listed in your SPF record, or one that falls outside a specified CIDR range, it may trigger an "IP in CIDR" error and be rejected or flagged as spam.
This mechanism helps prevent email spoofing by ensuring that only legitimate senders can use your domain. However, a misconfigured SPF record, particularly with incorrect CIDR ranges, can inadvertently block your own legitimate emails, leading to these types of bounce messages.
Common causes of IP in CIDR errors
Several factors can lead to an "IP in CIDR" error, often stemming from DNS misconfigurations or unexpected changes in your sending infrastructure. It's crucial to investigate these potential causes to restore your email deliverability.
Common causes of IP in CIDR errors
Incorrect SPF record: Your SPF record might not accurately list all the IP addresses or CIDR ranges from which your domain sends email. This is the most frequent cause. If your email service provider (ESP) changes their sending IPs, or you add a new sending source, your SPF record needs to be updated. An improperly configured SPF record can lead to your emails being rejected by receiving servers. Learn more about what SPF is and how it functions.
Shared IP addresses: If you're using a shared IP address provided by an ESP, and other users on that IP engage in suspicious activity, the entire CIDR block (or a portion of it) could be blocklisted. This could lead to a sudden influx of "IP in CIDR" errors, even if your own sending practices are legitimate. Receiving mail servers, such as Google for example, use a variety of IP ranges for their services.
Recipient's internal policies: Some mail servers might have strict internal policies or custom blocklists (blacklist) that reject emails from specific CIDR ranges, even if your SPF is correct. This is often seen with smaller ISPs or organizations that maintain their own stringent email filtering rules.
Dynamic IP addresses: If you're sending from a dynamic IP address (common for residential or small business internet connections), it may belong to a CIDR range typically associated with consumer use and thus deemed less trustworthy by mail servers.
Reverse DNS (rDNS) issues: While not directly a CIDR issue, improper rDNS (the reverse lookup of an IP address to a hostname) can lead to IP reputation problems that prompt receiving servers to apply stricter CIDR-based filtering.
These causes highlight the interplay between your DNS records, IP reputation, and the filtering rules of recipient mail servers. A proactive approach to monitoring and configuration is essential.
Diagnosing and resolving IP in CIDR errors
Diagnosing and resolving an "IP in CIDR" error requires a methodical approach, starting with examining the bounce message itself.
How to fix this issue
Examine the bounce message: The bounce message usually contains the exact error code, the sending IP address, and sometimes even the specific CIDR range causing the issue. This information is invaluable for pinpointing the problem. You might find more specific information on what "IP in CIDR" means.
Verify your SPF record: Check your domain's SPF record to ensure all current sending IP addresses and email service providers are correctly included using appropriate CIDR notation. Pay close attention to any include mechanisms to avoid exceeding the 10 DNS lookup limit. Incorrect SPF records are a common cause of deliverability issues. Ensure your SPF record is syntactically correct and covers all legitimate sending sources.
Example SPF record
This SPF record authorizes IP addresses within the 192.0.2.0/24 CIDR range and includes Mailchimp as an authorized sender.
Check IP blacklists/blocklists: Use a reliable blocklist checker to see if your sending IP or its associated CIDR range is listed. If it is, follow the delisting procedures for each blocklist. Some well-known blocklists (or blacklists) include the Spamhaus Policy Blocklist (PBL), which lists non-MTA IP addresses.
Contact the recipient's postmaster: If the issue persists and your configurations appear correct, reach out to the postmaster of the receiving domain (e.g., postmaster@recipient-domain.com) with details of the bounce and your sending IP. They might be able to provide more insight into their specific filtering rules or even whitelist your IP.
Resolving these issues is often a collaborative effort between you, your ESP, and the recipient's email administrator.
Preventing future CIDR-related issues
To prevent future "IP in CIDR" errors and maintain strong email deliverability, implement the following best practices.
Best practices for email deliverability
Regularly review SPF, DKIM, and DMARC: Ensure your email authentication records are always up-to-date and correctly configured. Any change in your sending infrastructure, like a new ESP or dedicated IP, requires an update. This is fundamental to avoiding errors like "IP not authorized".
Monitor your IP reputation: Keep an eye on your IP reputation and proactively address any blocklist (blacklist) listings. Services that monitor blocklists can alert you to issues before they severely impact deliverability.
Warm up new IPs: If you acquire new dedicated IP addresses, gradually increase your sending volume to build a positive sending history. This helps establish trust with recipient mail servers. A sudden burst of email from a cold IP can trigger spam filters.
Maintain good sender practices: Consistently send relevant, wanted emails to engaged recipients. High complaint rates, low engagement, and sending to invalid addresses can damage your reputation and lead to stricter filtering, including CIDR-based blocks.
Shared vs. dedicated IP addresses
Choosing the right type of IP address for your sending needs impacts how your emails are perceived by recipients.
Shared IPs: Cost-effective, but your reputation is tied to other senders on the same IP. Can be affected by others' poor sending practices. Suitable for low-volume senders or those just starting out.
Dedicated IPs: Full control over your sending reputation. Requires a warm-up period and consistent sending volume. Ideal for high-volume senders or those with very sensitive deliverability requirements. This can help prevent issues related to dedicated IPs on certain blocklists.
By proactively managing your sending infrastructure and adhering to email best practices, you can significantly reduce the likelihood of encountering "IP in CIDR" errors and ensure your emails reach their intended recipients.
Views from the trenches
Best practices
Always keep your SPF records updated, especially when changing email service providers or adding new sending IPs.
Regularly check your sending IP address and its associated CIDR ranges against major email blocklists.
Implement DMARC policies at `p=quarantine` or `p=reject` to protect your domain and guide receiving servers on how to handle unauthenticated mail.
Maintain clean email lists to reduce bounces and avoid spam traps that can lead to IP blacklisting.
For new dedicated IPs, always follow a proper IP warm-up schedule to build a positive sending reputation over time.
Common pitfalls
Neglecting to update SPF records when your ESP changes its sending IP addresses or adds new IP ranges.
Not monitoring your email bounce logs for specific error messages like 'IP in CIDR' that indicate deeper issues.
Sending high volumes of email from a new, unwarmed-up dedicated IP, triggering spam filters.
Failing to investigate the underlying reasons for 'IP in CIDR' errors, assuming it's solely the recipient's problem.
Using generic rDNS records that don't match your domain, which can negatively impact IP trustworthiness.
Expert tips
Look into the specific SMTP transaction point where the rejection occurs (e.g., after EHLO, after DATA). This can offer critical clues about the exact rule being triggered.
When dealing with selective blocking, examine the content and address collection processes of affected senders. Targeted blocks often indicate a perceived issue with sender behavior.
If an ISP suddenly implements new blocking rules, it may explain widespread 'IP in CIDR' errors without any changes on your end.
Verify that reverse DNS (rDNS) is correctly configured for all your sending IPs, as this is a fundamental trust factor for many mail servers.
Consider if the 'IP in CIDR' error is conditional. Some receiving servers apply rules based on content, recipient engagement, or historical sender behavior.
Expert view
Expert from Email Geeks says the IP in CIDR error likely relates to how your reverse DNS is configured.
2023-02-14 - Email Geeks
Expert view
Expert from Email Geeks says they might have just implemented a new rule to stop mail from certain IPs or ranges.
2023-02-14 - Email Geeks
Summary
An "IP in CIDR" error is a specific signal that your email's sending IP address is not aligning with the recipient mail server's expectations for authorized senders. While it can initially seem daunting, these errors are typically resolvable by diligently reviewing and correcting your DNS authentication records, particularly SPF.
The key is to understand that mail servers use CIDR to define trusted and untrusted IP ranges. When your email's source falls outside the expected range, it's flagged. Proactive monitoring, adherence to email authentication standards (like SPF, DKIM, and DMARC), and responsive troubleshooting are essential steps to ensure your messages reliably reach the inbox.
By addressing these technical details and maintaining healthy sending practices, you can minimize deliverability issues and ensure a smoother email experience for your recipients.
