SPF Flattening Explained: What It Is And Why It Matters For Email Deliverability

SPF flattening has become an essential technique for organizations managing complex email-sending infrastructures. As businesses increasingly rely on multiple third-party platforms—such as marketing tools, CRMs, support systems, and cloud email providers—their SPF records often grow long, fragmented, and prone to exceeding DNS lookup limits. When this happens, SPF authentication can fail, causing legitimate emails to be rejected or sent to spam. SPF flattening resolves these challenges by converting complicated SPF records full of includes into a single, simplified list of approved IP addresses, ensuring consistent and reliable authentication.

Understanding why SPF flattening matters begins with recognizing the crucial role SPF plays in email deliverability and security. Is your SPF flattened? SPF is one of the core email authentication mechanisms that helps receiving servers verify whether an email truly originates from the domain it claims to represent. However, when SPF breaks due to too many lookups, errors like permerror can severely harm sender reputation and inbox placement. By reducing DNS lookups and eliminating nested includes, SPF flattening helps organizations maintain SPF compliance, improve authentication outcomes, and protect their domain from spoofing and email-based threats.

Understanding SPF Records and Their Role in Email Authentication

Email authentication is fundamental in protecting businesses and individuals from spoofing, phishing, and unauthorized use of domain names. The Sender Policy Framework (SPF) record is a DNS TXT record that specifies which mail servers are permitted to send email on behalf of your domain. When an incoming mail server receives a message, it checks the SPF record published in the sender domain’s DNS. This check involves an interrogation of the allowed IP addresses and any affiliated mechanisms or include statements to determine sender verification.

SPF records typically contain mechanisms such as `ip4`, `ip6`, `include`, and `a` to define the servers and IP address ranges authorized to send mail. By specifying verified email sources—be they internal mail servers or external platforms like Google, Office 365, SendGrid, or CRMs handling marketing automation and customer support—you can drastically reduce the risk of spoofed messages passing as legitimate. Strong SPF management and adherence to SPF limitations are foundational for robust email delivery, sender verification, and domain SPF reputation.

The Challenges of Standard SPF Records: DNS Lookup Limits and Errors

While the SPF protocol is essential for sender verification and email deliverability, it introduces some unique challenges. Chief among these is the SPF mechanism limit—often called the DNS lookup limit. By RFC specification, a single SPF verification process may trigger a maximum of 10 DNS lookups (including all `include`, `a`, `mx`, `ptr`, and `exists` mechanisms), regardless of how many different mechanisms are included or how many nested records are present.

Exceeding the SPF lookup limits can cause errors such as the infamous Too Many Lookups Error (`permerror`), resulting in SPF errors that may cause legitimate emails to be flagged, increase the risk of email bounce, or disrupt mailflow monitoring in tools like Delivery Center. Nested records, which occur when one or more `include` mechanisms reference other domains with their own SPF records, can unwittingly multiply the number of DNS lookups far beyond what is permitted.

This issue is compounded when organizations rely on numerous third-party senders—for marketing, customer support, or order fulfillment—leading to the aggregation of many IP addresses and even overlapping IP ranges in their SPF configuration. The end result is a fractured SPF record that may break SPF compliance for sender verification and undermine email deliverability.

What Is SPF Flattening? How the Process Works

SPF flattening addresses these common challenges by simplifying the structure of an SPF record. Rather than using multiple `include` mechanisms and nested records referencing third-party senders, SPF flattening replaces all `include` statements with a direct list of IP addresses or IP address ranges that each referenced SPF record resolves to. The result is a flattened SPF record: a single, streamlined DNS entry containing just the necessary IP addresses for all approved email senders.

How SPF Flattening Operates

• Resolution of Includes and Mechanisms: The SPF flattening process starts by resolving all include mechanisms and referenced nested records. This recursive lookup is necessary to compile every IP address permitted by each include.
• Generation of IP List: Once all valid IP addresses and IP address ranges are collected (including those from Google, Office 365, SendGrid, and internal mail servers), the SPF flattening tool rewrites the SPF record as a flat list of `ip4` and `ip6` mechanisms.
• SPF Record Rewriting and Replacement: The original SPF record is replaced with the flattened SPF record, dramatically reducing the total DNS lookups since there are no more includes or indirect references. This tackles the SPF lookup limits head-on, mitigating the risk of hitting the Too Many Lookups Error and improving overall SPF compliance.

Types of SPF Flattening

• Manual SPF Flattening: Administrators use SPF record flattening tools or online services like MxToolbox SuperTool to resolve and rewrite SPF records manually. While this provides granular control, it requires vigilant monitoring and periodic manual SPF updates whenever third-party senders add or remove IP addresses.
• Automatic SPF Flattening: Automated solutions such as PowerDMARC ffer dynamic SPF management by automatically monitoring, flattening, and updating your SPF record in real-time. This ensures the flattened SPF record always reflects current sending infrastructure, helping maintain compliance with SPF lookup limits and simplifying SPF management.

Benefits and Potential Drawbacks of SPF Flattening

Key Benefits

• Guaranteed SPF Compliance: By eliminating nested records, includes, and excess DNS lookups, SPF flattening ensures your domain SPF remains compliant with protocol-imposed SPF limitations. This means fewer SPF errors such as the Too Many Lookups Error, reducing the risk of email bounce and maximizing email delivery success.
• Improved Email Deliverability: ISPs, spam filters, and mailflow monitoring services like Delivery Center rely on error-free authentication for sender verification. A flattened SPF record minimizes lookup errors, making it easier for receiving servers to authenticate verified senders, leading to better inbox placement and improved email deliverability.
• Simplified SPF Management: Whether performed via manual SPF updates or automatic monitoring through an SPF service or API, SPF flattening centralizes the list of approved IP addresses. This makes it easier to update SPF records and improves oversight, especially for organizations using multiple third-party senders such as CRMs, marketing automation, or customer support platforms.

Potential Drawbacks

• Maintenance Challenges: The static nature of manual SPF flattening means that changes to the IP addresses of included senders (like Google, SendGrid, or other cloud services) won’t reflect unless you update the SPF record proactively. Missed updates can result in failed sender verification and email bounce.
• Potential for Overlapping or Duplicate Senders: Aggregating IP address ranges from multiple email service providers can sometimes result in overlapping or duplicate entries within your SPF record. These redundancies make the record harder to manage and increase the chances of misconfigurations. As a result, SPF management becomes more complex and more prone to errors that may disrupt authentication.
• Record Length Limitations: DNS imposes strict limits on record length, including 255 characters per string and a 512-byte limit for UDP responses. When an SPF record is excessively flattened, it can become too long and exceed these constraints. This often requires additional optimization or splitting the record into manageable segments to maintain proper functionality.

Best Practices for Implementing and Maintaining Flattened SPF Records

Choose the Right SPF Management Approach

Evaluate your organization’s needs—whether you require real-time SPF updates (using automatic SPF flattening and dynamic SPF management) or if your configuration is static enough for manual SPF updates. For high-change environments, opt for SPF flattening tools or services that provide automatic monitoring and real-time updates, such as PowerDMARC, to keep up with evolving sender infrastructure.

Leverage SPF Record Flattening Tools

Use established solutions like MxToolbox SuperTool or PowerDMARC’s SPF Flattening Tool to resolve and compile all the IP addresses in your domain SPF. This ensures your flattened SPF record is both accurate and efficient. Consider using the API functionality provided by some SPF services for seamless, automated update workflows.

Regularly Review and Update SPF Records

Even a flattened SPF record can eventually become outdated. Set reminders or use automated monitoring to trigger updates whenever new third-party senders are added or existing providers change their IP ranges. Regular updates ensure the record remains accurate and compliant. This ongoing maintenance helps prevent SPF lookup errors and supports consistent authentication.

Watch for Overlaps, Duplicates, and Record Bloat

Carefully manage the addition of new email senders and IP addresses to your SPF record. Make sure to review the flattened record for duplicate entries or overlapping IP ranges that could lead to SPF errors. These issues can create configuration problems and weaken authentication reliability. Using SPF flattening tools that automatically identify such conflicts helps keep your SPF record accurate and optimized.

Integrate With Broader Email Authentication

Finally, remember that SPF flattening is just one aspect of a strong authentication framework. Pair SPF with DMARC and BIMI for brand protection, sender verification, and advanced email deliverability strategies. Integrate SPF compliance monitoring with your delivery center or mailflow monitoring platform for comprehensive oversight.

Effective SPF flattening ensures that your SPF record enforces sender authentication without falling foul of the SPF mechanism limit—keeping your organization’s emails delivered, authenticated, and protected against increasingly sophisticated email-borne threats.