What are the best practices for using SPF flatteners and managing SPF records?

Key findings

• Lookup limit evasion: SPF flatteners effectively help domains stay within the 10-DNS-lookup limit, preventing authentication failures.
• Dynamic vs. static flattening: Automated, dynamic SPF flattening services are generally more effective than one-off manual flattening, as they update records to reflect changes in included SPF policies.
• Interim solution: Flatteners can serve as a temporary fix for SPF issues while an organization works on long-term infrastructure changes or optimization strategies.
• Single point of failure: Relying on a third-party flattener introduces a potential single point of failure. If the service experiences downtime, it can impact email deliverability.
• DMARC considerations: The use of flatteners can be risky, particularly when a DMARC policy is set to p=reject, as failures can lead to legitimate emails being blocked.

Key considerations

• Service reliability: If using a flattener, opt for a reputable, paid service with a strong Service Level Agreement (SLA) to minimize risks of downtime or outdated records.
• Long-term strategy: View SPF flatteners as a temporary measure. The ultimate goal should be to optimize your SPF record manually or utilize subdomains for different sending purposes.
• Correct domain usage: Ensure that SPF records are configured for the correct domain (the RFC5321.From or Return-Path domain) rather than solely the main domain, especially for bulk email sending.
• Regular monitoring: Even with a flattener, regular checks of your SPF record are essential to confirm its validity and to prevent issues such as the SPF TempError.
• Avoid unnecessary includes: Many ESPs do not require an SPF include on your main domain if they use their own return paths. Removing such unnecessary includes can often bring your record within the lookup limit naturally.

Key opinions

• Effectiveness: Many marketers find SPF flatteners to be a practical and effective solution for immediate issues with exceeding the 10-lookup limit.
• Ease of use: Some flatteners are praised for being hands-off and not requiring frequent manual intervention after initial setup.
• Interim solution preference: Flatteners are often seen as a necessary interim fix, particularly when immediate infrastructure changes are not feasible.
• Concerns about free tools: There is a general sentiment that free SPF flattener tools might lack the reliability and support needed for critical email infrastructure.
• Misinformation from ESPs: Marketers frequently encounter ESPs recommending SPF includes for domains that don't actually require them, leading to unnecessary lookups. This aligns with findings on why ESPs recommend incorrect SPF records.

Key considerations

• Dynamic updates: When choosing a flattener, prioritize services that dynamically update the SPF record, rather than providing a static, one-time flattened version, as IP ranges can change.
• Long-term optimization: While flatteners are useful, marketers should still strive to optimize their SPF records by consolidating senders and using subdomains where appropriate, which helps with managing SPF 'a' records and the 10-lookup limit.
• Understanding return paths: A critical step is to understand which domain is actually used for the RFC5321.From (return-path) address for each ESP. Many ESPs use their own domains for bounces, eliminating the need for their SPF includes on your main domain. More details can be found on SPF record optimization.
• Paid solutions: For critical sending infrastructure, investing in a reliable, paid SPF management solution is often recommended over free tools to ensure stability and support.

Key opinions

• Workaround, not ideal: SPF flatteners are acknowledged as a necessary workaround for the 10-lookup limit, but experts caution that they are not the ideal long-term solution due to potential issues.
• Risk of single point of failure: A significant concern is that relying on a third-party flattener introduces a single point of failure. If the service fails, it can severely impact email authentication.
• Dynamic vs. static: Experts recommend using dynamic flattening services that automatically update IP addresses, as static flatteners can quickly become outdated, leading to deliverability problems.
• Strategic optimization: The preferred long-term approach is to optimize the SPF record directly, for instance, by consolidating senders or using subdomains, as opposed to relying solely on flattening. More information about this can be found at Sendmarc's SPF Optimization guide.
• Cost vs. benefit: While free tools exist, experts often advise paying for robust solutions with Service Level Agreements (SLAs) for enterprise-level reliability.

Key considerations

• Impact on DMARC: If you have an enforcing DMARC policy, SPF flattening failures can lead to legitimate emails being quarantined or rejected. Always ensure your DMARC, SPF, and DKIM records are in sync.
• Source domain awareness: Correctly identifying which domain (e.g., Return-Path) each ESP uses is crucial for accurate SPF configuration, avoiding unnecessary includes that contribute to the lookup limit. This is a common issue when troubleshooting SPF authentication issues.
• Proactive management: Regularly review and maintain your SPF record. Even with flatteners, changes in your sending infrastructure or ESPs can necessitate updates.
• Consider alternatives: Before implementing a flattener, explore consolidating email sending services or delegating sending to subdomains to reduce the complexity of your SPF record naturally.

Key findings

• Lookup limit defined: RFC 7208 explicitly states that during SPF evaluation, a DNS query mechanism (e.g., a, mx, ptr, exists, or include) must not cause more than 10 DNS lookups.
• Void lookups: Documentation specifies that void lookups (queries that return no records or an error) also count towards the 10-lookup limit, which can exacerbate issues even with seemingly simple records.
• TXT record limit: DNS records, including SPF TXT records, have a maximum length. While not directly SPF's 10-lookup limit, a flattened record's length could exceed this, leading to a CharacterStringTooLong error.
• Purpose of SPF: SPF (Sender Policy Framework) is designed to specify which IP addresses are authorized to send email on behalf of a domain, primarily for the MAIL FROM (RFC5321.From) address, not necessarily the visible From address.

Key considerations

• Minimize lookups: Documentation implicitly encourages minimizing the number of mechanisms that trigger DNS lookups to avoid exceeding the limit, promoting efficient record design.
• Avoid deprecated mechanisms: RFCs recommend against using mechanisms like ptr, which are inefficient and often lead to lookup failures or timeouts, impacting SPF evaluation. This is particularly relevant for issues like the Microsoft SPF DNS timeout.
• Flat record maintenance: While not explicitly in RFCs, the nature of flattened records (containing hardcoded IPs) requires diligent and frequent updates to remain accurate as included services change their IP ranges.
• Subdomain delegation: Documentation indirectly supports the use of subdomains for specific email sending purposes, allowing for separate, simpler SPF records that are less likely to hit the lookup limit for the main domain. This is covered in best practices for ESPs regarding SPF.