When is SPF flattening necessary for email authentication?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jul 2025
Updated 16 Aug 2025
7 min read
When managing email deliverability, one of the more technical aspects you might encounter is Sender Policy Framework (SPF). SPF is a critical email authentication protocol designed to prevent email spoofing by verifying that incoming mail from a domain comes from a host authorized by that domain's administrators. It's like a bouncer at a club, ensuring only authorized servers can send emails on your behalf.
However, SPF records aren't without their complexities, especially for organizations using multiple email service providers (ESPs). A common issue arises with the SPF 10-lookup limit, which can lead to legitimate emails failing authentication if your record is too complex. This is where SPF flattening often comes into the conversation.
Many ask if SPF flattening is truly necessary, particularly when using several services. We often hear conflicting advice on this topic. Understanding when and why to use SPF flattening, along with its potential drawbacks, is crucial for maintaining good email deliverability and protecting your domain.
The SPF specification (RFC 7208) imposes a limit of 10 DNS lookups for an SPF record. This means that during the evaluation of your SPF record, if a mail receiver needs to perform more than 10 DNS queries to resolve all include, a, mx, ptr, and exists mechanisms, it will result in an SPF PermError. This error effectively means the SPF record cannot be properly evaluated, often leading to emails being rejected or sent to the spam folder.
The rationale behind this limit is to prevent denial-of-service (DoS) attacks on DNS servers, which could occur if SPF records were allowed to trigger an unlimited number of DNS queries. It also helps speed up the authentication process. However, for organizations that use many third-party email services, reaching this limit is a common challenge.
Exceeding the 10-lookup limit can severely impact your email deliverability. When an email fails SPF authentication due to too many DNS lookups, it signals to recipient mail servers that the email's legitimacy cannot be verified, increasing the likelihood of it being marked as spam or outright blocked. This is why understanding the SPF lookup limit is essential for reliable email communication.
What is SPF flattening?
SPF flattening is a technique that aims to reduce the number of DNS lookups in your SPF record. It works by resolving all the include mechanisms to their underlying IP addresses or CIDR ranges. These IPs are then directly added to your SPF record, replacing the domains that previously required lookups.
Consider an SPF record that includes multiple ESPs, each with its own include statement. Each of these includes counts towards your 10-lookup limit, and some may even have nested includes that add more lookups. SPF flattening bypasses these recursive lookups by directly listing the IP addresses, which don't count towards the limit.
Before flattening
DNS lookups: Each include mechanism requires a DNS lookup. Nested includes can quickly exhaust the limit.
Record example:v=spf1 include:_spf.mail.com include:_spf.another.net ~all
After flattening
DNS lookups: Replaces include mechanisms with IP addresses, eliminating the need for lookups.
Record example:v=spf1 ip4:192.0.2.1 ip4:198.51.100.0/24 ~all
When SPF flattening is necessary
SPF flattening becomes necessary primarily when your SPF record exceeds the 10-DNS lookup limit. This typically happens to larger organizations or those that utilize numerous third-party email services for various functions, such as marketing, transactional emails, CRM, or support.
For instance, if you use Google Workspace, a marketing automation platform, and a customer support system, each requiring an SPF include, you can quickly hit or surpass this threshold. If you're experiencing SPF PermErrors in your DMARC reports or seeing emails land in spam despite having SPF set up, a bloated SPF record is a likely culprit.
When to consider flattening
Exceeding the limit: If your current SPF record has more than 10 DNS lookup mechanisms. You can use an email deliverability tester to check your record.
Multiple ESPs: If you rely on several third-party senders, such as Amazon SES, SendGrid, or an email marketing platform, their collective SPF includes can quickly add up.
Performance concerns: A complex SPF record with many lookups can sometimes introduce slight delays in email processing, though this is usually minor.
Before resorting to flattening, it's essential to audit your existing SPF record. Sometimes, organizations include unnecessary include statements from services no longer in use, or from those that do not actually send email on your domain's behalf. Removing these can often bring your record within the 10-lookup limit without the need for flattening.
Risks and alternatives to SPF flattening
While SPF flattening can solve the 10-lookup limit problem, it introduces its own set of challenges. The primary drawback is maintenance. Email service providers frequently update their sending IP addresses. If you flatten your SPF record, you are essentially hardcoding those IPs into your DNS. When an ESP changes its IP ranges, your flattened SPF record becomes outdated, leading to legitimate emails failing SPF authentication.
This means you would need a robust system to constantly monitor changes in your ESPs' IP addresses and update your SPF record accordingly. Manual updates are prone to human error and can be time-consuming, especially for large organizations with many sending services. Automation is almost a necessity for this approach.
Instead of flattening, a more robust solution often involves leveraging DMARC (Domain-based Message Authentication, Reporting & Conformance). DMARC works with both SPF and DKIM (DomainKeys Identified Mail) for email authentication. If either SPF or DKIM passes alignment, DMARC will pass. This means you don't necessarily need every sender to pass SPF, as long as it passes DKIM.
Method
Pros
Cons
SPF flattening
Solves the 10-lookup limit directly.Simplifies DNS queries for mail receivers.
Requires constant manual updates if IPs change.Risk of legitimate emails failing authentication if not updated.Can violate some ESP terms if not handled carefully.
DMARC with DKIM
Provides comprehensive email authentication.Resilient to SPF 10-lookup issues if DKIM passes.Offers visibility into email sending with reports.
Requires proper DKIM setup for all senders.Initial setup can be more complex than basic SPF.
Maintaining healthy SPF records
The key to effective email authentication and deliverability is not necessarily SPF flattening, but rather proactive management of your DNS records and a holistic approach to email security. This includes regularly reviewing your SPF record for unnecessary entries and ensuring your DKIM and DMARC records are correctly configured and aligned.
For organizations with a large number of email senders, focusing on optimizing your SPF record by removing redundant entries is often the first and most effective step. Beyond that, a robust DMARC implementation, ensuring that your legitimate emails pass either SPF or DKIM authentication (or both), provides a more resilient solution against spoofing and deliverability issues without the maintenance overhead of SPF flattening.
While some services offer automated SPF flattening, relying solely on them can be risky due to the dynamic nature of IP addresses. It's crucial to understand how ESPs manage their SPF records and to prioritize robust, long-term strategies for email authentication.
Views from the trenches
Best practices
Always start by auditing your existing SPF record to remove any unnecessary or outdated entries, as this can often resolve lookup limit issues.
Implement DMARC alongside SPF and DKIM. DMARC's flexibility means that if your SPF record is complex, DKIM can still pass authentication, ensuring deliverability.
Monitor your DMARC aggregate reports regularly. These reports provide invaluable insight into your email authentication status, helping identify SPF failures.
If using SPF flattening, ensure you have an automated solution to update your record as ESPs frequently change their IP ranges, which can break your flattened SPF.
Common pitfalls
Ignoring the SPF 10-lookup limit, which leads to 'PermError' failures and reduces email deliverability, often sending legitimate emails to spam.
Over-relying on SPF flattening without understanding the ongoing maintenance it requires, resulting in outdated SPF records and authentication failures.
Including SPF mechanisms for services that do not send emails on your domain's behalf, needlessly increasing your lookup count.
Failing to implement DKIM or DMARC, which would provide additional layers of authentication and resilience against SPF limitations.
Expert tips
Consider prioritizing DKIM authentication for your major email senders. If DKIM aligns and passes, DMARC will pass, even if SPF has issues.
For complex environments, a comprehensive email deliverability platform can help manage SPF, DKIM, and DMARC records dynamically, minimizing manual intervention.
When troubleshooting, use a reliable SPF record checker to identify lookup count issues and syntax errors before implementing changes.
Remember that SPF is just one part of a robust email security posture; combine it with DKIM and DMARC for best results.
Marketer view
Marketer from Email Geeks says they tested an SPF flattening service and were told it was unnecessary, leading to confusion about its actual need for domains with multiple services.
2022-02-08 - Email Geeks
Marketer view
Marketer from Email Geeks says they shared their flattened and unflattened SPF records, noting they were using services like amazonses.com and sendgrid.net, which often contribute to lookup issues.
2022-02-08 - Email Geeks
Key takeaways for SPF management
SPF flattening can be a necessary solution when your domain's SPF record hits or exceeds the 10-DNS lookup limit, particularly if you manage a complex email ecosystem with numerous third-party sending services. While it effectively addresses the PermError caused by excessive lookups, it introduces the critical need for constant monitoring and updates due to dynamic IP addresses.
Ultimately, the decision to flatten SPF should be part of a broader email authentication strategy that includes a well-configured DMARC policy and robust DKIM signing. Focusing on these foundational elements often provides a more resilient and less maintenance-intensive path to ensuring your emails reach the inbox reliably.
Google Workspace, a marketing automation platform, and a customer support system, each requiring an SPF include, you can quickly hit or surpass this threshold. If you're experiencing SPF PermErrors in your DMARC reports or seeing emails land in spam despite having SPF set up, a bloated SPF record is a likely culprit.
Amazon SES, 
SendGrid, or an email marketing platform, their collective SPF includes can quickly add up.
