Summary
Discrepancies in SPF (Sender Policy Framework) validation results across different tools, such as MXToolbox reporting an SPF record as "too thick" while other tools show a higher score, are a common source of confusion for email senders. This often stems from varying interpretations of SPF mechanisms, particularly the 10 DNS lookup limit as defined in RFC 7208. Some tools may count certain SPF mechanisms (like exists lookups) more strictly or have different internal algorithms for evaluating complex SPF records, leading to differing assessments of their validity and impact on deliverability.
Key findings
- Differing interpretations: MXToolbox is known for its strict interpretation of SPF records, often flagging records that exceed the 10 DNS lookup limit as problematic. Other tools might be more lenient or use different calculation methods.
- Impact of exists mechanism: The exists mechanism in an SPF record can be a particular point of divergence, as some tools may incorrectly handle or count these lookups.
- DNS lookups: Every include, a, mx, and ptr mechanism (and exists) contributes to the DNS lookup count, which, if exceeded, can cause SPF validation failures.
- Postmaster warnings: Receiving postmaster warnings about SPF issues, despite high scores from some tools, indicates that the stricter interpretation (like MXToolbox's) is often closer to how receiving mail servers actually evaluate SPF.
Key considerations
- Prioritise strict validation: When tools conflict, err on the side of caution and address warnings from stricter validators like MXToolbox. This aligns better with how many ISPs and email providers enforce SPF.
- Reduce lookups: Consolidate SPF records and minimize the number of include mechanisms. Consider using SPF flattening services if necessary, though this comes with its own set of considerations. For more information, read our guide on managing DNS TXT record length limits.
- Understand tool methodologies: Familiarize yourself with how different SPF lookup tools operate. Some may provide detailed breakdowns of DNS lookups (like MXToolbox's DNS tab), which can help identify specific issues.
- Monitor DMARC reports: Ultimately, DMARC reports provide the most accurate picture of how your SPF (and DKIM) records are performing at scale across various mailboxes. Check your SPF record using an online checker to ensure proper configuration.
- Address specific mechanisms: If a specific mechanism, such as exists, is causing issues, explore alternative configurations or consult with your ESP or DNS provider to optimize your SPF record.
What email marketers say
Email marketers often encounter conflicting information when validating their SPF records, leading to confusion about the actual health of their email authentication. These discrepancies can stem from a lack of deep technical understanding of SPF mechanisms and how different validation tools interpret them. The primary concern for marketers is ensuring emails reliably reach the inbox, which requires adhering to the most stringent interpretations of SPF to avoid deliverability issues.
Key opinions
- Conflicting reports are common: Marketers frequently observe SPF scores differing between tools, making it challenging to pinpoint actual problems versus tool-specific interpretations. A guide on conflicting authentication results can provide further insight.
- Postmaster feedback is key: Despite high scores from some validators, direct feedback from postmaster teams (like Dotdigital) indicating issues with an SPF record is a strong indicator that the record needs attention, as their systems are the ultimate arbiters.
- Prioritize inbox placement: The ultimate goal is to avoid the spam folder. If any tool, especially one known for its rigor like MXToolbox, suggests a problem, it's worth investigating to prevent deliverability issues.
- Complexity of SPF: Understanding the nuances of SPF, such as how include and exists mechanisms contribute to the DNS lookup count, is crucial for marketers trying to maintain good deliverability.
Key considerations
- Trust stricter tools: When in doubt, assume the stricter tool (like MXToolbox) is providing more accurate feedback on potential issues that receiving servers might encounter. Learn more about boosting email deliverability rates.
- Simplify SPF records: Aim for the simplest possible SPF record that covers all legitimate sending sources to avoid hitting the 10 DNS lookup limit. This often involves reducing unnecessary include statements.
- Consult ESP documentation: Refer to your Email Service Provider's (ESP) official documentation for their recommended SPF configuration. They often provide optimized records that minimize lookups.
- Regular monitoring: Continuously monitor your SPF (and DKIM/DMARC) records using a combination of tools and email sending guides to catch any changes or issues quickly.
- DMARC for comprehensive view: Implement DMARC to gain visibility into how your SPF and DKIM authentication is performing across the email ecosystem, which helps in identifying real-world deliverability impacts.
Email marketer from Email Geeks questions the discrepancy between SPF validation tools, noting that MXToolbox flagged their SPF as "too thick" while another tool showed a high score, and a postmaster team confirmed it would cause issues. This highlights the confusion that arises from differing interpretations of SPF records.
Email marketer from AutoSPF suggests that SPF record checks are essential for email validation. Using tools like MXToolbox can help ensure only authorized mail servers are permitted to send emails for a domain, preventing spoofing and improving deliverability.
What the experts say
Deliverability experts consistently warn about the potential for SPF records to exceed the 10 DNS lookup limit, a common issue that stricter tools like MXToolbox accurately detect. They emphasize that while some tools might provide a more forgiving score, the actual email receiving infrastructure often adheres to the RFC standards, which are less forgiving. Therefore, understanding the nuances of SPF mechanisms, particularly how exists mechanisms are counted, is paramount for maintaining optimal email deliverability.
Key opinions
- MXToolbox accuracy: Many experts lean towards MXToolbox being more accurate in flagging SPF records as "too thick" because its strict evaluation often mirrors how receiving mail servers interpret the 10 DNS lookup limit.
- The exists mechanism: The exists mechanism is a specific point of concern, as some validation tools may not process it correctly or count it towards the DNS lookup limit in the same way, leading to false positives in other tools.
- DNS tab insights: Experts advise checking the DNS tab in tools like MXToolbox to see a detailed breakdown of all lookups being performed. This transparency helps in identifying which specific mechanisms are contributing to the higher count. Read more about how broken SPF records affect deliverability.
- Ongoing tool development: Some SPF validation tools are continuously updated to better handle complex SPF records and adhere more closely to RFC standards, suggesting that issues with certain mechanisms might be resolved in future versions.
Key considerations
- Adhere to RFC standards: Always prioritize adherence to RFC 7208 regarding the 10 DNS lookup limit, as this is what most mail exchangers follow. For more details, consult resources on SPF TempError.
- Optimize SPF records: Regularly review and optimize your SPF record to minimize DNS lookups. This might involve consolidating multiple include mechanisms where possible or using tools that help flatten SPF records without exceeding the limit.
- Use multiple validation tools: While MXToolbox is often correct, using several reputable SPF validation tools can provide a more comprehensive view and help identify consistent issues.
- Stay informed about RFC updates: Keep up to date with the latest RFCs and best practices in email authentication to ensure your setup remains compliant and effective. A helpful source for such information is SpamResource.
- Collaborate with providers: Work closely with your ESPs and DNS providers to ensure they understand the implications of SPF record complexity and can assist in maintaining an optimal configuration.
Deliverability expert from Email Geeks explains that an SPF lookup involving exists:%{i}._spf.mta.salesforce.com can be problematic because the code within some SPF validation tools may not handle this mechanism correctly, contributing to lookup count issues.
Deliverability expert from Email Geeks suggests checking the DNS tab within tools like MXToolbox to view all the DNS lookups being performed for an SPF record. This detailed view often confirms that MXToolbox's stricter assessment of SPF validity is likely correct.
What the documentation says
Official documentation and RFCs provide the definitive rules for SPF record construction and validation. RFC 7208, the current specification for SPF, explicitly outlines constraints such as the 10 DNS lookup limit. Discrepancies between validation tools often arise from varying levels of compliance with these strict guidelines or different approaches to counting lookups, especially for complex mechanisms like exists queries. Adhering closely to these documented standards is critical for robust email authentication.
Key findings
- RFC 7208 compliance: The SPF specification (RFC 7208) clearly defines that SPF processing must not require more than 10 DNS lookups that return a record. This limit includes a, mx, ptr, and exists mechanisms.
- Void lookups: The RFC also mentions void lookups, which occur when a DNS query results in a NXDOMAIN or NOERROR response with no answers. Too many of these can also lead to issues, though they are distinct from the 10-lookup limit.
- Error types: SPF validation can result in Pass, Fail, SoftFail, Neutral, None, TempError, or PermError. A "too thick" SPF record typically results in a PermError due to exceeding the DNS lookup limit.
- Dynamic SPF management: Some documentation outlines methods for dynamic SPF record management, such as using SPF flattening or a dedicated service, to help domains stay within the lookup limits while including multiple sending sources.
Key considerations
- Strict adherence: Always ensure your SPF record strictly adheres to the 10 DNS lookup limit as defined in RFC 7208. Exceeding this limit will likely result in a PermError from receiving mail servers.
- Avoid redundancy: Do not include redundant mechanisms or unnecessary include statements in your SPF record. Each one counts towards the limit, even if they point to the same or previously covered IPs.
- Implement DMARC: DMARC leverages both SPF and DKIM. Even if your SPF record is complex, DMARC can provide insight into authentication results. A simple guide to DMARC, SPF, and DKIM can help solidify your understanding.
- Consult RFCs: For definitive answers on SPF behavior, refer directly to the RFC 7208 documentation. This is the authoritative source for how SPF should be implemented and evaluated.
The Internet Engineering Task Force (IETF) in RFC 7208 specifies that SPF validation must not involve more than 10 DNS lookups that return a record. This includes lookups for a, mx, ptr, and exists mechanisms.
The RFC further details that if an SPF record requires more than 10 DNS lookups to fully evaluate, the result of the SPF check should be a PermError. This hard failure ensures that overly complex records do not lead to undefined behavior or resource exhaustion on receiving mail servers.
