Why does MXToolbox report SPF as too think while other tools show a higher score?
Michael Ko
Co-founder & CEO, Suped
Published 5 May 2025
Updated 15 Aug 2025
6 min read
It can be confusing when different email tools provide conflicting reports about your Sender Policy Framework (SPF) record. You might see MXToolbox indicate your SPF record is too thin, while other validators report a high score, perhaps 9/10 or even 10/10. This discrepancy often leads to frustration and uncertainty about your email deliverability, especially when postmaster teams raise concerns.
The core of this issue usually lies in how different tools interpret the SPF DNS lookup limit. The SPF specification (RFC 7208) states that SPF processing must not perform more than 10 DNS lookups that resolve to an IP address. Exceeding this limit results in a PermError, which can severely impact your email's ability to reach the inbox.
Understanding why MXToolbox stands out in its strict reporting is crucial for maintaining good email hygiene and ensuring your messages are authenticated correctly. This guide will clarify the reasons behind these differing scores and provide actionable insights to resolve any underlying SPF issues.
The SPF specification imposes a limit of 10 DNS lookups to prevent denial-of-service attacks and ensure efficient processing of SPF records. Each time an SPF mechanism like a, mx, ptr, exists, or include requires a DNS query to determine an IP address, it counts towards this limit. If your SPF record triggers more than 10 such lookups, a PermError is returned, meaning the SPF record cannot be fully evaluated. This often leads to legitimate emails failing SPF authentication and potentially landing in the spam folder.
MXToolbox is known for its strict adherence to this 10-lookup limit. When it reports your SPF record as too thin (or too many lookups), it's almost always because your record, or one of the records it includes, exceeds this threshold. Other tools, however, might use different methodologies or even SPF flattening techniques that can make a complex record appear simpler to their checks, leading to a seemingly higher score. This doesn't mean the underlying issue is resolved, only that their checker isn't flagging it.
The impact of SPF PermError
When an SPF record hits a PermError, the receiving mail server is unable to definitively verify if the sending server is authorized. This often leads to messages being rejected outright or, more commonly, being marked as spam. It directly impacts your email deliverability and sender reputation, making it a critical issue to address.
The exists mechanism is a common culprit in SPF records that appear too thin. Although valid, the exists mechanism requires a DNS lookup and some SPF validation tools may not process it correctly or count it against the 10-lookup limit in the same way that MXToolbox (or a mail server) would. This can lead to a false sense of security regarding your SPF record's compliance.
Resolving the
Why other tools show a higher score
Some SPF validation tools aim to provide a quick, simple assessment, potentially overlooking the nuances of the 10-lookup limit or the complexities introduced by mechanisms like exists. They might implement SPF flattening automatically, which converts include mechanisms into IP addresses to reduce lookups. While this can make the record appear shorter, it introduces its own set of challenges, as the SPF record is no longer dynamically updated by the service provider.
The key distinction often boils down to strict RFC compliance versus a more lenient, practical assessment. MXToolbox provides a very accurate, real-world view of how a receiving mail server would process your SPF record. If MXToolbox flags an issue, it's highly probable that some mail servers will encounter the same problem, leading to deliverability failures. Discrepancies between different deliverability tools are common and highlight the importance of understanding the underlying protocols.
MXToolbox
Strict RFC Compliance: Adheres rigorously to the 10-DNS lookup limit set by the SPF specification.
Accurate Error Reporting: Flags PermErrors accurately, which can lead to emails being rejected.
Mechanism Interpretation: May not handle complex mechanisms like exists optimally, leading to a too thin warning if it exceeds lookups.
Other SPF tools
Varied Compliance: May not always strictly enforce the 10-lookup limit or may employ flattening.
Potentially Misleading Scores: Can show higher scores even if a PermError would occur in a real-world scenario.
Simpler Interpretations: Might simplify complex SPF records for easier reading, which can hide underlying issues. For example, Google Postmaster Tools can fluctuate
Strategies for SPF optimization
Too thin SPF issue
The primary goal is to ensure your SPF record adheres to the 10-lookup limit. Here are some strategies to achieve this:
Consolidate includes: Many email service providers (ESPs) offer a single include mechanism that covers all their sending IPs, rather than requiring multiple ones. Consolidate where possible to reduce lookups. For example, some ESPs might offer include:spf.example.com that covers numerous underlying IPs.
Remove unnecessary mechanisms: Review your SPF record and remove any mechanisms (like ptr) that are no longer needed or are redundant.
Direct IP addresses: If an ESP provides a static list of IP addresses, consider using ip4 or ip6 mechanisms directly instead of includes. Note that direct IPs require manual updates if the ESP's IPs change.
Beware of SPF flattening: While some services offer SPF flattening to bypass the 10-lookup limit, it's not a true solution. It can lead to stale records if an ESP changes its IP ranges, potentially causing emails to fail SPF authentication down the line.
Regularly monitor your SPF record, especially after adding new sending services or updating existing ones. Tools like MXToolbox will continue to be invaluable for identifying potential issues, including those related to the DNS lookup limit. A correctly configured SPF record is a foundational element for strong email deliverability.
Views from the trenches
Best practices
Always validate your SPF record with multiple tools, but prioritize those that adhere to RFC standards like MXToolbox.
Aim to keep your SPF record concise and under the 10-DNS lookup limit to avoid PermErrors and improve deliverability.
Regularly review your SPF record when adding or removing email sending services.
Use `include` mechanisms from ESPs that consolidate their IPs into a single lookup where possible.
Common pitfalls
Overlooking the 10-DNS lookup limit, which can lead to legitimate emails failing SPF authentication.
Relying solely on tools that employ SPF flattening, as this can hide underlying issues and lead to stale records.
Including unnecessary mechanisms like `ptr` that can increase DNS lookups without providing significant benefit.
Not removing old `include` statements for services no longer in use, contributing to a bloated SPF record.
Expert tips
Consider setting up DMARC reporting to gain visibility into SPF authentication failures and understand how mailbox providers are evaluating your emails.
If using multiple third-party senders, explore SPF record optimization services that can help manage complex records within limits.
Prioritize the most frequently used sending sources in your SPF record to minimize the impact of reaching the lookup limit.
Educate your team on the importance of proper SPF configuration to prevent accidental record changes that could break authentication.
Expert view
Expert from Email Geeks says that one of the SPF lookups containing exists:%{i}._.spf.mta.salesforce.com can cause problems because the SPF validation code doesn't handle the `exists` mechanism well. This often leads to inaccurate reports from some tools.
October 20, 2020 - Email Geeks
Expert view
Expert from Email Geeks says that MXToolbox is likely correct in its assessment regarding the SPF record being too thin. Users should check the DNS tab in MXToolbox to see all the lookups being performed.
October 20, 2020 - Email Geeks
Ensuring robust email authentication
Dealing with discrepancies between SPF validation tools can be confusing, but it's important to trust the tool that adheres most strictly to the SPF specification. MXToolbox provides a rigorous check that accurately reflects how major mailbox providers (like Mailgun or Microsoft) would evaluate your record.
If MXToolbox flags your SPF record as too thin due to exceeding the 10-lookup limit, it's a genuine issue that needs addressing, regardless of what other tools indicate. Ignoring it can lead to SPF PermErrors, causing your emails to be flagged as spam or rejected entirely.
By actively managing and optimizing your SPF record, ensuring it stays within the defined limits, you can significantly improve your email deliverability and avoid common authentication pitfalls. Proactive monitoring and adherence to best practices are key to ensuring your messages reach their intended recipients reliably.
Postmaster Tools can fluctuate 
Microsoft) would evaluate your record.
