What Is SPF Flattening? A Solution For
Exceeding DNS Lookup Limit


The Sender Policy Framework is a method used to authenticate emails, aimed at blocking email spoofing by identifying the mail servers permitted to send messages for a specific domain. A notable drawback of SPF records is the restriction on DNS lookups; surpassing this limit can result in SPF failures, which may affect the delivery of emails. To address this, SPF flattening is employed to streamline SPF records by minimizing the required DNS lookups. 

This article will delve into SPF flattening, the reasons behind the DNS lookup limit, its effects on email deliverability, and strategies for effectively implementing SPF flattening.


Understanding SPF and the DNS Lookup Limit


What is SPF?

SPF is an email authentication method that relies on the Domain Name System to assist mail servers in confirming whether incoming emails originate from a legitimate source. This process involves examining the SPF record in the sender's DNS settings. The SPF record includes a list of authorized IP addresses and domains allowed to send emails for that particular domain.


The DNS Lookup Limit in SPF

To avoid overwhelming the DNS system, SPF enforces a restriction of 10 DNS lookups for each SPF record. Should an SPF record surpass this threshold, it triggers a PermError (permanent error), which leads to the failure of SPF validation. This failure can result in emails being rejected or marked as spam.


Why Do SPF Records Exceed the DNS Lookup Limit?


  • Use of Various Third-Party Email Providers: Companies frequently rely on several external services, such as Google Workspace, Microsoft 365, and email marketing tools, which necessitate their SPF entries.

  • Layered Includes: SPF records can include mechanisms to link to other SPF records. However, each inclusion can add more lookups, rapidly depleting the available limit.

  • Domain-Based IP Address Specification: Some services use domain names instead of directly listing IP addresses, resulting in extra DNS lookups needed to resolve these domains.


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What is SPF Flattening?


SPF flattening is a method designed to minimize DNS lookups in an SPF record by substituting domain-based queries with specific IP addresses. Rather than relying on numerous include statements that lead to an overload of DNS requests, this technique consolidates those includes into their respective IP addresses and incorporates them directly into the SPF record.


How SPF Flattening Works

  • Convert Domains to IP Addresses: Begin by obtaining all IP addresses linked to the domains specified in the SPF record.

  • Substitute Includes with IP Addresses: Rather than referencing include:_spf.example.com, incorporate the resolved IP addresses directly into the SPF record.

  • Streamline for Brevity and Effectiveness: The simplified SPF record should be kept brief while ensuring it remains valid.

Example of SPF Flattening

Before Optimization:

v=spf1 include:_spf.google.com include:_spf.microsoft.com include:mailservice.com ~all

Every included domain generates extra DNS queries.

Following Optimization:

v=spf1 ip4:192.168.1.1 ip4:192.168.2.2 ip4:203.0.113.5 ip4:203.0.113.6 ~all

This process removes the need for DNS queries, avoiding SPF record failures caused by lookup restrictions.


Benefits of SPF Flattening


  • Avoids SPF Lookup Limit Issues: By minimizing the number of lookups, SPF records remain compliant with the 10-lookup cap.

  • Boosts Email Deliverability: A properly configured SPF record lowers the likelihood of emails being flagged as spam or bounced back.

  • Optimizes Performance: Cutting down on unnecessary DNS queries leads to quicker response times and enhances the efficiency of email authentication processes.

  • Strengthens Security: Less reliance on external SPF allows organizations to maintain greater control over their email authentication configurations.

Challenges and Considerations


IP Address Changes

A significant disadvantage of SPF flattening is that the IP addresses of third-party email providers can vary. Because the flattened SPF record consists of fixed IPs, it requires frequent updates to stay current with these changes.



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Manual vs. Automated SPF Flattening

  • Manual SPF Optimization: It's feasible to manage and revise SPF records manually, but it demands ongoing supervision.

  • Automated SPF Management Solutions: Numerous online platforms and services are available that can automatically refresh SPF records, helping to maintain their accuracy and compliance with limits.

Alternative Solutions to SPF Flattening


  • Employing SPF Macros: Utilizing SPF macros such as (%{i}, %{s}, etc.) enables the dynamic evaluation of SPF records.

  • Dividing SPF Records: Certain organizations choose to divide their SPF records among different subdomains, which helps in spreading out the lookup requests.

  • Utilizing DKIM and DMARC: While SPF is essential, integrating it with DKIM and DMARC enhances the overall security of email communications. Learn more by visiting this link.