Summary
Overstuffed SPF records, particularly those exceeding the 10 DNS lookup limit, pose a significant challenge for email deliverability. While the SPF specification clearly outlines this limit, many internet service providers (ISPs) like Gmail, Hotmail, and Yahoo often appear more forgiving in practice, sometimes still passing SPF results for non-compliant records. This discrepancy creates a dilemma for senders, who must decide whether to adhere strictly to the specification or rely on observed ISP behavior. Common approaches to address this include SPF flattening, which consolidates multiple DNS lookups into a single record, and the strategic use of subdomains to segregate sending platforms and reduce the number of required includes per domain. Each approach presents its own set of complexities and considerations, from dynamic updates to management overhead.
Key findings
- ISP leniency: Major ISPs frequently process SPF records that exceed the 10-lookup limit without apparent failure, suggesting a more forgiving approach than the strict specification. This can make diagnosis difficult when using standalone SPF validation tools that strictly enforce the limit.
- Flattening as a solution: SPF flattening is a widely discussed method for consolidating multiple SPF includes into a single IP address list, effectively reducing DNS lookups to zero for the primary record. This requires careful management to ensure the flattened record remains up to date. AutoSPF's blog discusses this in detail.
- Subdomain strategy: Utilizing separate subdomains for different sending platforms or mail streams allows for more granular SPF records, each tailored to a specific set of sending IPs, thus minimizing the need for extensive includes on a single domain. URIports Blog highlights this approach.
- DKIM's role: Many organizations rely heavily on DKIM for authentication, sometimes even overlooking SPF issues due to its perceived robustness and flexibility when SPF becomes overly complex.
Key considerations
- Deliverability risk: While some ISPs are forgiving, relying on this leniency is a dangerous assumption. Overstuffed SPF records can lead to intermittent delivery failures and misclassification as spam, impacting overall email deliverability.
- Management complexity: Implementing SPF flattening or subdomain strategies adds a layer of DNS management complexity. Flattened records require dynamic updates when included IP ranges change, and subdomain usage needs careful planning, especially for small clients.
- RFC adherence: The 10 DNS lookup limit is a standard defined in RFC 7208. Adhering to standards ensures broader compatibility and predictability across various mail receivers. Understanding the importance of this limit is crucial.
What email marketers say
Email marketers often face a tough choice when dealing with SPF records that exceed DNS lookup limits. While many notice that major ISPs are surprisingly lenient, relying on this leniency feels risky. The consensus leans towards adopting technical workarounds like SPF flattening or segmenting email streams by subdomain. However, these solutions introduce their own set of challenges, particularly for smaller clients or complex sending setups involving multiple platforms. Marketers frequently express frustration with the rigid SPF specification in contrast to the dynamic nature of modern email ecosystems, where numerous third-party services require their own SPF includes.
Key opinions
- ISP forgiveness is a reality: Many marketers observe that large providers like Gmail, Hotmail, and Yahoo don't strictly enforce the 10 DNS lookup limit, with SPF checks often passing despite overstuffing. This can create a false sense of security.
- Subdomains are a viable path: Using dedicated subdomains for different sending services helps contain SPF record complexity. This allows each subdomain to have a minimal SPF record, avoiding the lookup limit on the primary domain. This strategy also aligns with best practices for email deliverability and subdomain usage.
- Flattening is a common workaround: SPF flattening services or manual processes are frequently used to replace includes with IP addresses, circumventing the lookup limit. However, this method requires vigilance as included IP ranges can change.
- DKIM provides a fallback: Many marketers increasingly rely on DKIM for authentication, especially when SPF becomes too cumbersome or prone to issues due to the DNS lookup limit. This shift reduces the dependency on a perfectly configured SPF record.
Key considerations
- Client constraints: For smaller clients, implementing multiple subdomains for different sending platforms might seem overly complex or 'silly', despite being a technically sound solution. It adds administrative overhead that smaller operations may struggle with.
- Reply management: Using subdomains can complicate reply management if an organization needs all replies directed to a single primary email address, such as support@example.com. Alias domains (like with Google for Business) can help alleviate this.
- Opaque flattening: While effective, SPF flattening can make the record's origins opaque. When includes are replaced by raw IP addresses, it becomes harder to trace which IP belongs to which sending service without external documentation. Marketers should understand when flattening is necessary.
- Bad ESP guidance: Some email service providers provide incorrect guidance, leading clients to publish SPF records for the wrong domain or to overstuff records unnecessarily. Marketers must ensure proper SPF authentication setup with multiple ESPs.
Marketer view
An email marketer from Email Geeks states that major email providers like Gmail, Hotmail, and Yahoo seem to pass SPF results for overstuffed records without issues, indicating they are more forgiving than the SPF specification suggests. This behavior simplifies things for senders, as they do not encounter immediate failures for non-compliant SPF records.
Marketer view
A marketer from Email Geeks notes that many of their clients rely solely on DKIM for email authentication due to the complexities and limitations associated with SPF, especially when dealing with the DNS lookup limit. They also mention that SPF flattening is a viable alternative.
What the experts say
Experts in email deliverability acknowledge the persistent challenge of SPF records exceeding DNS lookup limits. While the specification (RFC 7208) is clear on the 10-lookup rule, practical implementation by major mail receivers often demonstrates a more lenient, though inconsistent, approach. Experts emphasize that relying on this observed leniency is risky and not a substitute for adherence to the standard. They highlight the IETF's (Internet Engineering Task Force) stance on potential SPF revisions, noting that changes to fundamental aspects like the lookup limit face significant hurdles due to backward compatibility concerns and a perceived lack of overwhelming community demand for a new SPF version. The current recommendation for senders remains strategic mitigation techniques rather than awaiting a protocol update.
Key opinions
- Specification adherence: The SPF specification's 10 DNS lookup limit is a hard rule that should ideally be followed, despite observed ISP behavior. Non-compliance, even if currently tolerated, introduces an element of risk to deliverability.
- IETF's position on revision: The IETF (Internet Engineering Task Force) has considered and rejected proposals to change the SPF lookup limit recently. The primary reasons include concerns about backward compatibility, the existing SPF RR type, and a lack of sufficient community interest to warrant a new SPF version. This discussion can be seen in the spfbis mailing list archives.
- Receiver discretion: Major email receivers like Google have the operational capability to bypass the SPF lookup limit if they choose to, which they sometimes do. However, this is their internal policy, not an official relaxation of the standard for all senders or receivers.
- SPF's historical context: SPF was developed at a time when email sending infrastructures were simpler. The current landscape, with numerous cloud-based ESPs, pushes the limits of its original design. Some experts feel SPF was always ugly from the start, a stopgap until DMARC could be fully developed.
Key considerations
- Inconsistent evaluation risk: If the SPF lookup limit were removed from the specification, records that currently fail would then pass. This would lead to a period of highly inconsistent evaluations as mail servers slowly adopt the new standard, creating greater uncertainty for senders.
- Practical workarounds are necessary: Given the unlikelihood of a quick SPF spec revision, solutions like SPF flattening (which might include addressing character string limits) and subdomain delegation remain the primary, most robust options for senders to ensure compliance and optimal deliverability. Best practices for SPF flatteners should be followed.
- DMARC as the long-term solution: While SPF issues persist, DMARC is seen as the more mature and comprehensive email authentication protocol, providing better alignment and reporting capabilities. Future efforts in email authentication are focused on DMARC's evolution, with SPF serving as a foundational component.
Expert view
A deliverability expert from Email Geeks indicates that many clients choose to rely on DKIM as their primary authentication method because of recurring issues with SPF record complexity and the DNS lookup limit. They also suggest SPF flattening as a robust alternative to manage these challenges effectively.
Expert view
An expert from Email Geeks states that the SPF specification's lookup limit, originating from an earlier era, should be a primary candidate for revision. However, they express doubt about the likelihood of the SPF specification being revisited in the near future given current priorities and community dynamics.
What the documentation says
The official documentation and RFCs pertaining to SPF (Sender Policy Framework) clearly define the mechanisms for authenticating email senders and establish specific limitations, most notably the 10 DNS lookup limit. RFC 7208, the current SPF specification, details the evaluation process, including how DNS queries for 'include', 'a', 'mx', 'ptr', and 'exists' mechanisms contribute to this limit. Exceeding this limit results in a 'PermError' during SPF evaluation. While the specification is explicit, the practical enforcement by various mail receivers can vary. Documentation often suggests best practices for managing complex sending environments, emphasizing the importance of efficient record construction to avoid hitting these predefined boundaries.
Key findings
- The 10-lookup limit: RFC 7208, Section 4.6.4, explicitly states that SPF processing MUST NOT perform more than 10 mechanisms that require DNS lookups. These include 'a', 'mx', 'ptr', 'exists', and 'include' mechanisms.
- PermError consequence: If the DNS lookup limit is exceeded, the SPF evaluation result is a 'PermError' (Permanent Error). This indicates a problem with the SPF record itself, which can lead to legitimate emails being rejected or marked as spam by receiving servers.
- TXT record size limit: While distinct from the lookup limit, DNS TXT records also have a character string length limit (typically 255 characters per string, with multiple strings concatenated). Overstuffed SPF records can also hit this limit, resulting in validation issues. Senders should understand how to manage DNS TXT record length limits.
- No ptr mechanism: RFC 7208 discourages the use of the ptr mechanism due to its high computational cost and potential for large numbers of DNS lookups, further emphasizing the need to minimize lookups.
Key considerations
- Careful mechanism selection: To stay within the 10-lookup limit, administrators must be selective with mechanisms used in their SPF records. Prioritizing 'ip4' and 'ip6' mechanisms over 'include', 'a', or 'mx' when possible can significantly reduce DNS query overhead.
- Using 'redirect' vs. 'include': While both 'redirect' and 'include' count as one DNS lookup, 'redirect' effectively replaces the current SPF record with the target one, which can simplify management for a single external SPF record. Understanding how SPF 'a' records affect lookups is also important.
- Macros and dynamic SPF: Some advanced solutions, often referred to as 'hosted SPF' or 'dynamic SPF,' use SPF macros to create a single entry that dynamically resolves to the necessary IP addresses without exceeding the 10-lookup limit on the domain's own DNS record. Proofpoint's Hosted SPF documentation explains this.
- Troubleshooting TempError: A TempError in SPF results indicates a transient DNS error, but repeated TempErrors can also signal an underlying issue such as an excessively complex SPF record that causes timeouts during validation.
Technical article
RFC 7208 specifies that an SPF record must not cause more than 10 DNS lookups when evaluated. This limit is critical to prevent denial-of-service attacks and ensure efficient processing by mail receivers.
Technical article
The Microsoft Exchange Online documentation states that SPF records that exceed the DNS lookup limit will result in a PermError, which indicates an invalid configuration. This can lead to emails failing authentication checks and being treated as suspicious.
Related resources
2 resources
Related pages
How do SPF 'a' records affect DNS lookups and the 10-lookup limit, and what are the best practices?
How do broken SPF records, like those with too many DNS lookups or exceeding size limits, affect email deliverability and authentication?
Why is my SPF record too long and how to manage DNS TXT record length limits?
How to format SPF TXT records, add domain includes, and avoid DNS size issues?
What are the best practices for DNS lookups, SPF records, and subdomain usage for email deliverability?
How to resolve 'CharacterStringTooLong' error when flattening SPF records?
How important is the 10 DNS lookups limit on SPF records?
What are the best practices for using SPF flatteners and managing SPF records?
When is SPF flattening necessary for email authentication?
Demystifying the SPF TempError in your DMARC reports
