How complex is the SPF spec for building an SPF checking library?

Summary

The complexity of the SPF specification stems from several factors, including its intricate syntax, mechanisms, modifiers, interactions with DNS records, and the existence of corner cases, macros, and PTR records. The 10 DNS lookup limit, complicated by nested includes, poses a significant challenge, particularly for organizations using multiple third-party services. This limit, along with potential syntax errors and record length exceeding 255 characters, makes implementation and maintenance difficult. Managing multiple domains and services adds further complexity. Recommendations include careful planning, optimization, using tools to analyze and validate SPF records, SPF flattening, and limiting DNS lookups.

Key findings

  • Complex Specification: The SPF specification has complex elements like syntax, mechanisms, modifiers, DNS interactions, corner cases, macros and PTR records that make robust implementation challenging.
  • DNS Lookup Limit: The 10 DNS lookup limit and nested 'include' statements make it easy to exceed the limit, especially for orgs with multiple services.
  • Implementation Difficulty: Implementing and maintaining SPF correctly is difficult because it's challenging to manage all the limits and syntax correctly.
  • Record Limitations: SPF records can only be 255 characters long.

Key considerations

  • Careful Planning: Proper planning and optimization are essential for effective SPF management and to ensure deliverability.
  • Use Validators: Validate SPF records to ensure they are correct and do not exceed limits.
  • Limit Lookups: Reduce the number of DNS lookups to avoid issues.
  • SPF Flattening: Implement SPF flattening to consolidate records and reduce the number of DNS lookups.
  • Address Corner Cases: When building an SPF checking library, account for all potential corner cases.

What email marketers say
10Marketer opinions

The complexity of SPF implementation arises from several factors. While the basic concept of SPF is straightforward, practical application is challenging due to the 10 DNS lookup limit, which is further complicated by nested includes. Managing SPF records becomes difficult with multiple third-party email services, and debugging SPF records can also be challenging. Exceeding the DNS lookup limit or having records longer than 255 characters can lead to deliverability issues. Therefore, careful planning, optimization, and the use of tools or SPF flattening techniques are recommended.

Key opinions

  • DNS Lookup Limit: The 10 DNS lookup limit presents a significant hurdle in SPF implementation, especially for organizations using numerous third-party services.
  • Nested Includes: Nested 'include' statements in SPF records contribute to the DNS lookup count, making it easier to exceed the limit.
  • Record Length: SPF record length exceeding 255 characters can lead to validation failures and deliverability problems.
  • Debugging Challenges: Debugging SPF records can be tricky, making it important to use validators.

Key considerations

  • Plan and Optimize: Careful planning and optimization are essential to manage SPF records effectively.
  • Use Tools: Employ tools to analyze and optimize SPF records to stay within the DNS lookup limit and ensure correct syntax.
  • SPF Flattening: Consider SPF flattening techniques to reduce the number of DNS lookups.
  • Record size: Reducing the record size will help prevent SPF validation failures and improve deliverability.
Marketer view

Email marketer from Mailhardener explains that while the basic concept of SPF is straightforward, the 10 DNS lookup limit creates significant challenges for larger organizations or those using multiple third-party email services, needing careful management of includes and lookups to stay within the limit.

April 2022 - Mailhardener
Marketer view

Email marketer from StackOverflow explains that SPF can be tricky to implement due to the 10 DNS lookup limit and the difficulties managing nested include statements. He also shares that debugging SPF records can be challenging.

August 2022 - StackOverflow
Marketer view

Email marketer from ServerFault explains that SPF looks simple on the surface but has a lot of subtle complexities that make robust implementation difficult. He references the complexity of correctly handling all of the edge cases for resource limits and DNS lookup failures

June 2023 - ServerFault
Marketer view

Marketer from Email Geeks says SPF macros are evil.

February 2023 - Email Geeks
Marketer view

Email marketer from Reddit explains that SPF records are limited to 10 DNS lookups and that nested lookups count towards that limit and its common to see records that break this. They suggest ensuring the record is correct using online validators.

June 2024 - Reddit
Marketer view

Email marketer from AuthSMTP explains that SPF can become complex as businesses use more cloud services, each requiring inclusion in the SPF record. This increases the risk of exceeding the 10 DNS lookup limit, requiring careful management and potentially using alternative solutions like SPF flattening.

November 2024 - AuthSMTP
Marketer view

Email marketer from EasyDMARC shares that SPF record length should be under 255 characters. Exceeding this limit can cause issues. They recommend reducing the record size where possible to avoid SPF validation failures and improve deliverability.

February 2022 - EasyDMARC
Marketer view

Marketer from Email Geeks says SPF macros are one of the reasons why he never wanted to open that box and asks Steve if he will share it on github someday.

November 2021 - Email Geeks
Marketer view

Email marketer from Word to the Wise explains that SPF records can become complex due to the limitations, specifically the 10 DNS lookup limit. This limit requires careful planning and potentially flattening of the SPF record to ensure deliverability.

November 2022 - Word to the Wise
Marketer view

Email marketer from DMARC Analyzer explains the 10 DNS lookup limit in SPF records can easily be exceeded, causing deliverability issues. They recommend using tools to analyze and optimize SPF records to stay within the limit.

August 2022 - DMARC Analyzer

What the experts say
3Expert opinions

Implementing an SPF checking library is complex due to the intricate nature of the SPF specification, which involves handling corner cases, macros, and PTR records. This complexity also makes SPF implementation and maintenance difficult, with the 10 DNS lookup limit being a significant source of errors.

Key opinions

  • SPF Spec Complexity: The SPF spec contains complex and unexpected elements, including corner cases, macros, and PTR records.
  • Implementation Challenges: Implementing and maintaining SPF correctly is a significant challenge.
  • DNS Lookup Limit: The 10 DNS lookup limit is a common source of errors in SPF configurations.

Key considerations

  • Account for Corner Cases: When building an SPF checking library, ensure it handles all the weird corner cases in the SPF spec.
  • Address DNS Lookup Limit: Be mindful of the 10 DNS lookup limit and its potential to cause errors during SPF implementation and maintenance.
  • Use Validators: Use validators to ensure SPF records are correct and not exceeding the DNS lookup limit.
Expert view

Expert from Email Geeks shares that when it's polished it'll be at <http://github.com/wttw/spf|github.com/wttw/spf>.

June 2021 - Email Geeks
Expert view

Expert from Email Geeks shares he spent weeks building an SPF checking library and found the SPF spec to be complex with weird corner cases, macros, and PTR records.

October 2022 - Email Geeks
Expert view

Expert from Word to the Wise explains that one of the biggest challenges in SPF is that it can be hard to implement and maintain correctly. It notes that the 10 DNS lookup limit is a common source of errors.

July 2024 - Word to the Wise

What the documentation says
5Technical articles

The SPF specification, while appearing straightforward, contains several complexities that can make robust implementation challenging. These complexities arise from various aspects, including syntax, mechanisms, modifiers, and interactions with other DNS records. A primary concern is the 10 DNS lookup limit, which is easily exceeded, particularly with nested 'include' mechanisms. To manage these complexities and maintain deliverability, advanced techniques such as SPF flattening and dynamic SPF solutions, as well as using tools for optimization, are recommended. Common errors like exceeding the lookup limit or syntax errors can lead to SPF failures, especially when managing multiple domains and email services.

Key findings

  • Specification Complexities: The SPF specification includes intricate syntax, mechanisms, modifiers, and DNS record interactions that add to implementation difficulty.
  • DNS Lookup Limit: Exceeding the 10 DNS lookup limit is a common issue, often caused by nested 'include' mechanisms, which can lead to SPF failures.
  • Multiple Domains: Managing SPF records becomes more complex when dealing with multiple domains and various email services.

Key considerations

  • Limit DNS Lookups: Limit the number of DNS lookups within an SPF record to avoid issues.
  • SPF Flattening: Consider using SPF flattening or dynamic SPF solutions to stay within the DNS lookup limit.
  • Use Optimization Tools: Employ tools to check and optimize SPF records to ensure they are correctly configured and to prevent exceeding the DNS lookup limit or having syntax errors.
Technical article

Documentation from RFC Editor explains that the SPF specification, while seemingly simple, has several complexities related to syntax, mechanisms, modifiers, and interactions with other DNS records which can make a robust implementation challenging.

April 2023 - RFC Editor
Technical article

Documentation from Google Workspace Admin Help explains that common errors in SPF records, such as exceeding the DNS lookup limit or having syntax errors, can lead to SPF failing. They also share the complexity can arise from managing multiple domains and services sending email.

October 2023 - Google
Technical article

Documentation from Microsoft recommends limiting the number of DNS lookups within an SPF record to avoid issues. They advise using tools to check and optimize SPF records and potentially consolidating SPF records where possible.

March 2024 - Microsoft
Technical article

Documentation from Valimail explains that the complexities of SPF, particularly around DNS lookups and the limitations they impose, necessitate advanced management techniques such as SPF flattening or using a dynamic SPF solution to maintain deliverability without exceeding the limits.

December 2024 - Valimail
Technical article

Documentation from KDMARC explains that a common issue with SPF records is exceeding the 10 DNS lookup limit. It says 'include' mechanisms cause nested lookups, and too many of these can cause the record to fail and explains ways to avoid this using tools.

March 2021 - KDMARC