When is SPF flattening necessary for email authentication?

Summary

SPF flattening is generally considered necessary when an SPF record approaches or exceeds the 10 DNS lookup limit, a restriction set to prevent denial-of-service attacks. This situation commonly arises when organizations utilize multiple email service providers (ESPs), third-party integrations, or complex SPF configurations. By simplifying the SPF record, flattening helps avoid authentication failures, maintains deliverability, and ensures compatibility with various email receivers. Proactive monitoring of DMARC reports and regular reviews of the SPF record are also recommended to identify and address potential SPF-related issues.

Key findings

  • Lookup Limit: The 10 DNS lookup limit, as defined by RFC 7208, is the primary driver for needing SPF flattening.
  • Multiple Sources: Using multiple email sending sources (ESPs, third-party services) increases the likelihood of exceeding the lookup limit.
  • Deliverability Impact: Exceeding the lookup limit can lead to SPF authentication failures, resulting in reduced email deliverability.
  • DMARC Monitoring: Monitoring DMARC aggregate reports helps identify SPF-related issues and the effectiveness of SPF flattening solutions.
  • Complexity Factor: Complex SPF records with numerous includes can also contribute to exceeding the lookup limit.

Key considerations

  • Record Assessment: Evaluate the complexity of the SPF record and the number of DNS lookups it requires.
  • Third-Party Impact: Carefully consider the number and configuration of third-party services integrated with the email setup.
  • Proactive Approach: Implement SPF flattening proactively, especially when adding or removing email sending services.
  • Regular Monitoring: Monitor DMARC reports to identify any SPF-related issues promptly.
  • Compatibility: Consider SPF flattening to ensure maximum compatibility with different email receivers and prevent potential deliverability problems.

What email marketers say
13Marketer opinions

SPF flattening is generally necessary when an SPF record approaches or exceeds the limit of 10 DNS lookups. This is a common issue when using multiple email service providers (ESPs) or third-party integrations. The process simplifies the SPF record, preventing authentication failures and deliverability problems. Monitoring DMARC reports is also recommended to identify and correct any SPF misconfigurations.

Key opinions

  • Lookup Limit: Exceeding the 10 DNS lookup limit in an SPF record is the primary reason for needing SPF flattening.
  • Multiple ESPs: Using multiple email service providers or third-party services significantly increases the likelihood of surpassing the lookup limit.
  • Deliverability Impact: Unflattened SPF records that exceed the lookup limit can lead to email authentication failures and reduced deliverability.
  • DMARC Monitoring: Monitoring DMARC reports is crucial to identifying SPF-related issues and assessing the effectiveness of SPF flattening.
  • Proactive Approach: Reviewing and flattening SPF records proactively is advisable, especially when adding or removing email sending services.

Key considerations

  • Record Complexity: Evaluate the complexity of your SPF record and the number of DNS lookups before implementing SPF flattening.
  • Third-Party Services: Consider the number and configuration of third-party services integrated with your email setup.
  • Monitoring: Implement continuous monitoring of DMARC reports to identify and address any SPF-related issues promptly.
  • Alternative Solutions: Explore alternative solutions like using a universal SPF record before resorting to SPF flattening.
  • Regular Reviews: Perform regular reviews of your SPF record to ensure it remains optimized and compliant with DNS lookup limits.
Marketer view

Email marketer from MailerLite clarifies that a poorly configured SPF record (e.g., exceeding lookup limits) can negatively impact email deliverability. SPF flattening can help maintain deliverability by simplifying the record.

May 2023 - MailerLite
Marketer view

Email marketer from DNSimple advises to review and flatten your SPF record proactively if you add or remove email sending services. This ensures the record remains optimized and compliant with the DNS lookup limit.

August 2022 - DNSimple
Marketer view

Email marketer from MXToolbox highlights that if tools like MXToolbox show that your SPF record exceeds 10 lookups, SPF flattening becomes necessary to ensure email deliverability.

December 2023 - MXToolbox
Marketer view

Marketer from Email Geeks suggests putting the universal SPF string in front of the original SPF policy to automatically protect the policy from too many DNS lookups and other potential errors.

December 2023 - Email Geeks
Marketer view

Email marketer from Mailhardener shares that SPF flattening helps to stay within the 10 DNS lookup limit, ensuring that emails don't fail SPF checks due to exceeding this limit. This is particularly useful when using multiple email service providers.

April 2022 - Mailhardener
Marketer view

Marketer from Email Geeks explains that SPF flattening is necessary if you're exceeding 10 DNS lookups. They also suggest monitoring DMARC aggregate reports before making any decisions with SPF flattening solutions, as many people get SPF wrong and have unnecessary includes that can be omitted.

June 2021 - Email Geeks
Marketer view

Email marketer from StackExchange notes that SPF flattening is important when the complexity of your SPF record (due to multiple includes) starts causing authentication failures because of exceeding the DNS lookup limit.

February 2023 - StackExchange
Marketer view

Email marketer from GlockApps shares that SPF flattening is required when your domain’s SPF record contains too many nested DNS lookups (more than 10). It simplifies the SPF record, preventing deliverability issues.

April 2023 - GlockApps
Marketer view

Email marketer from Email on Acid points out that using numerous third-party email marketing platforms and ESPs often necessitates SPF flattening, as each integration adds to the DNS lookup count.

November 2021 - Sinch Email
Marketer view

Email marketer from AuthSMTP explains that SPF flattening should be considered when you are using multiple email service providers, and your SPF record is complex, approaching the limit of 10 DNS lookups.

May 2021 - AuthSMTP
Marketer view

Email marketer from SendLayer indicates SPF flattening becomes crucial when you start encountering SPF "permerror" issues due to exceeding the maximum number of DNS lookups allowed, which can cause emails to be rejected.

February 2025 - SendLayer
Marketer view

Email marketer from Reddit mentions that in practice, if you use more than two or three external services (like SendGrid, Mailgun, etc.) in your SPF record, you'll likely need SPF flattening to avoid exceeding the lookup limit.

August 2021 - Reddit
Marketer view

Email marketer from SpamExperts suggests considering SPF flattening when you want to ensure maximum compatibility with different email receivers and to prevent potential deliverability problems stemming from SPF limitations.

October 2024 - SpamExperts

What the experts say
2Expert opinions

SPF flattening is essential when an SPF record nears or surpasses the 10 DNS lookup limit, typically due to multiple sending sources. This avoids SPF failures and maintains email deliverability.

Key opinions

  • DNS Lookup Limit: SPF flattening is required when DNS lookups in an SPF record exceed 10.
  • Multiple Senders: Organizations with multiple email sending sources commonly need SPF flattening.
  • Deliverability Issues: Failure to flatten SPF records approaching the limit can result in deliverability problems.

Key considerations

  • Assessment: Assess the number of sending sources and their impact on SPF record complexity.
  • Monitoring: Continuously monitor the SPF record and deliverability rates to identify issues.
  • Implementation: Implement SPF flattening proactively to prevent potential deliverability problems.
Expert view

Expert from Word to the Wise answers that SPF flattening is a useful tool when an organization has multiple sending sources and their SPF record approaches the 10 DNS lookup limit, which can cause deliverability issues.

January 2023 - Word to the Wise
Expert view

Expert from Spam Resource explains that SPF flattening becomes necessary when the number of DNS lookups required by your SPF record exceeds the limit of 10, leading to SPF failures and potential deliverability problems.

June 2022 - Spam Resource

What the documentation says
4Technical articles

SPF flattening is necessary when an SPF record exceeds or approaches the 10 DNS lookup limit imposed by RFC 7208. This limit, designed to prevent denial-of-service attacks, is often reached when organizations use multiple email sources (including third-party services). Flattening reduces the number of DNS lookups, preventing SPF validation failures and deliverability issues.

Key findings

  • RFC Limit: RFC 7208 mandates a maximum of 10 DNS lookups per SPF check, necessitating flattening when this limit is approached.
  • DDoS Prevention: The 10-lookup limit is in place to prevent denial-of-service attacks via excessive DNS queries.
  • Multiple Sources: Using multiple email sending sources, including third-party services, significantly increases the likelihood of exceeding the lookup limit.
  • Validation Failure: Exceeding the lookup limit can lead to SPF validation failures, causing deliverability problems.

Key considerations

  • Lookup Count: Regularly assess the number of DNS lookups required by your SPF record.
  • Third-Party Services: Carefully consider the number of third-party services that are included in your SPF record.
  • Proactive Flattening: Proactively flatten your SPF record if your organization uses multiple email sources and the lookup count approaches the limit.
  • Testing: Test your SPF record after flattening to ensure correct configuration and prevent unintended consequences.
Technical article

Documentation from EasyDMARC explains that SPF flattening becomes necessary when an SPF record exceeds the 10 DNS lookup limit. This limit, set to prevent denial-of-service attacks, can be problematic for organizations using multiple third-party email services.

June 2024 - EasyDMARC
Technical article

Documentation from Google Workspace Admin advises that if your domain sends email from more than one source (e.g., in-house servers and third-party senders), you should evaluate your SPF record. If the lookups approach 10, consider SPF flattening.

November 2023 - Google
Technical article

Documentation from DMARC Analyzer explains that SPF flattening is used to reduce the number of DNS lookups in an SPF record, which is crucial for preventing SPF validation failures when the limit of 10 lookups is exceeded.

May 2024 - DMARC Analyzer
Technical article

Documentation from RFC 7208 specifies that SPF implementations MUST limit the number of mechanisms and modifiers that cause DNS lookups to at most 10 per SPF check, which necessitates SPF flattening when this limit is reached.

July 2023 - RFC Editor