How complex is the SPF spec for building an SPF checking library?
Matthew Whittaker
Co-founder & CTO, Suped
Published 31 Jul 2025
Updated 18 Aug 2025
8 min read
When you think about the Sender Policy Framework (SPF), it often seems straightforward: a simple DNS TXT record listing authorized sending IP addresses. However, diving into the actual specification for building a robust SPF checking library quickly reveals a landscape far more intricate than initially perceived. The journey from a high-level understanding to a precise, RFC-compliant implementation uncovers numerous complexities that challenge even experienced developers.
The core of SPF lies in its DNS TXT record, identified by v=spf1. This record defines which servers are permitted to send email on behalf of a domain. Parsers must correctly identify this version string and then process a series of mechanisms and modifiers, each with specific evaluation rules and effects on the SPF result. The initial parsing stage, while seemingly simple, demands careful handling of whitespace, case sensitivity, and malformed entries, as even a single incorrect character can invalidate the entire record.
A significant hurdle for SPF checking libraries is managing DNS lookups. The RFC 7208 specification imposes a strict limit of ten DNS lookups during SPF evaluation to prevent denial-of-service attacks. Exceeding this limit results in a PermError. Implementing this check accurately means a library must track every DNS query, including those triggered by include, a, mx, and ptr mechanisms. Recursive lookups within include directives are particularly tricky, often leading to unintended lookup chains, which can result in an SPF PermError.
For more detail, consider reviewing our guide on how important the 10 DNS lookups limit is. It's a critical aspect of SPF implementation that frequently causes deliverability issues.
The 10 DNS lookup limit (PermError)
The SPF specification allows a maximum of 10 DNS lookups to resolve all mechanisms and modifiers in a record. Exceeding this limit triggers an SPF PermError, leading to authentication failures and potential email rejections or spam folder placement. This is a common pitfall in SPF record configuration, especially for organizations using multiple third-party email services.
Mechanism tracking: A library must meticulously count each DNS query initiated by SPF mechanisms like a, mx, and include.
Recursive resolution: When an include mechanism points to another domain's SPF record, all DNS lookups within that included record also count towards the limit.
Impact: A PermError due to excessive lookups indicates a misconfigured SPF record, signaling to recipient servers that the domain's email policy cannot be reliably verified.
Anatomy of an SPF record
Decoding SPF mechanisms and modifiers
The SPF specification includes a variety of mechanisms, each designed to specify authorized hosts or networks. These include a (matching A records), mx (matching MX records), ip4 and ip6 (for IP addresses/ranges), and the often problematic ptr mechanism. While ptr is deprecated due to its heavy reliance on reverse DNS lookups and performance issues, a comprehensive SPF checker still needs to handle it for older, non-compliant records. Each mechanism has specific evaluation rules, including qualifiers (+, -, ~, ?) that determine the policy's outcome.
Modifiers add another layer of complexity. The redirect modifier, for instance, tells a checker to retrieve the SPF record of another domain and use its mechanisms instead. This can lead to complex lookup chains and requires careful management of the DNS lookup limit. Similarly, the exp (explanation) modifier, while optional, means a library must be ready to fetch and present a human-readable explanation if an SPF check fails. This adds to the overall parsing and interpretation logic.
The include mechanism is particularly noteworthy for its role in increasing SPF record complexity. It allows a domain to delegate SPF checking to another domain's SPF record. While useful for integrating third-party email services, it can easily lead to PermError if not managed carefully, as each included domain contributes to the 10-DNS-lookup limit. Many organizations use SPF flatteners to mitigate this, adding another layer of consideration for library developers who might encounter flattened records.
Here's a breakdown of common SPF mechanisms and their complexity impact:
Mechanism
Description
Complexity Impact
a
Matches if the sender's IP address is an A record for the domain.
Requires DNS A record lookup. Counts as one lookup.
mx
Matches if the sender's IP address is an MX record for the domain.
Requires DNS MX and then A record lookups for each MX host. Can quickly exhaust lookup limit.
ptr
Matches if the sender's IP has a reverse DNS entry pointing to the domain. Deprecated.
Requires reverse DNS (PTR) and then A/AAAA lookups. Very costly and unreliable, yet must be supported.
include
Refers to another domain's SPF record for evaluation.
Recursively evaluates another SPF record, all its lookups count towards the limit. High complexity for lookup management.
exists
Matches if a specified domain name exists (has any DNS record).
Requires a DNS A record lookup. Can be used for complex conditional checks.
The enigmatic world of SPF macros
One of the most notoriously complex features of the SPF specification is the use of SPF macros. These allow for dynamic expansion of values within the SPF record, such as the sender's IP address, domain name, or local part of the email address. While powerful for creating flexible and context-aware SPF policies, macros introduce significant parsing and evaluation challenges for a library. They require intricate string manipulation, variable substitution, and a deep understanding of the various macro-expansion rules, as detailed in our comprehensive analysis on SPF macros.
For example, a macro like %{i} expands to the sender's IP address, while %{d} expands to the domain. The spec defines various transformations and delimiters that can be applied to these macros, such as reversing parts of a domain or IP, or extracting specific octets. A library needs to correctly parse these macro strings, perform the dynamic substitutions based on the current email context, and then evaluate the resulting string as if it were part of the original SPF record.
The dynamic nature of macros means that a library cannot simply pre-evaluate an SPF record. It must perform the macro expansion at the time of evaluation, using the specific IP address and domain information of the email being checked. This adds a layer of runtime processing that distinguishes SPF from simpler static record checks and makes it notoriously difficult to debug if not implemented correctly. Formatting SPF TXT records is crucial to avoid these types of complications.
Real-world challenges and edge cases
Beyond the explicit rules of RFC 7208, building a real-world SPF checker requires accounting for practical considerations and behaviors of mail servers. One common issue is the handling of void lookups, where a DNS query returns no records. The spec defines how these should be handled, but subtle differences in DNS resolver behavior or network conditions can lead to unexpected TempError or PermError results, even if the SPF record itself is technically valid. Microsoft's email systems, for instance, can introduce unique challenges with DNS timeouts.
Furthermore, SPF does not operate in a vacuum. It's one part of a larger email authentication ecosystem that includes DKIM and DMARC. While an SPF checking library might focus solely on SPF, it must acknowledge how its output will be consumed by DMARC policies. Misinterpretations of SPF results, especially regarding alignment requirements, can lead to DMARC failures. Understanding this broader context, as discussed in how SPF, DKIM, and DMARC work, is essential for ensuring the library's utility in real-world scenarios.
Finally, robustness against malformed or intentionally deceptive SPF records is paramount. Spammers and phishers constantly try to exploit weaknesses. A checking library must be resilient, returning correct error codes for invalid records and not crashing or misinterpreting ambiguous syntax. This requires extensive validation logic beyond simple parsing, ensuring that every edge case, from unexpected characters to recursive loops in include statements, is handled gracefully. Incorrect handling can lead to false positives, allowing unauthorized emails to pass SPF checks, or false negatives, blocking legitimate emails. When SPF unauthorized mail is prohibited, it signals a properly configured and enforced policy.
The contrasting views of SPF
User's perspective
Simplified View: Many users see SPF as just a v=spf1 record with a few IP addresses and includes. Their goal is simply to authorize sending sources.
Tool-Driven: Often relies on online generators and validators, abstracting away the underlying technicalities.
Focus on Outcome: Primarily concerned with whether emails pass authentication and land in the inbox, not the intricacies of how.
Developer's perspective
RFC Compliance: Must adhere strictly to every detail of RFC 7208, including subtle nuances for edge cases.
Dynamic Evaluation: Needs to handle macro expansion, recursive DNS lookups, and the 10-lookup limit dynamically.
Robust Error Handling: Must correctly identify and report PermError, TempError, and other states under varying network conditions.
The intricate nature of SPF validation
Building an SPF checking library from scratch is a significant undertaking that highlights the true complexity of the SPF specification. It goes far beyond simply reading a TXT record. From meticulously adhering to the RFC to handling dynamic macros, managing DNS lookup limits, and anticipating real-world email authentication quirks, the task demands a deep technical understanding and rigorous testing.
Ultimately, the robustness of an SPF checking library directly impacts email deliverability and security for users. A well-implemented library can accurately validate sending sources, helping to combat phishing and spoofing. Despite the inherent difficulties, the effort contributes to a more secure and reliable email ecosystem, underscoring why precise SPF validation is so critical.
Views from the trenches
Best practices
Always strictly adhere to RFC 7208 when parsing and evaluating SPF records.
Implement robust DNS caching and efficient lookup tracking to stay within the 10-DNS-lookup limit.
Thoroughly test your library against a wide range of valid, invalid, and malformed SPF records, including those with complex macro usage.
Common pitfalls
Underestimating the complexity of SPF macros and their dynamic evaluation.
Failing to account for the recursive nature of 'include' mechanisms and hitting the 10-DNS-lookup limit.
Not handling deprecated mechanisms like 'ptr' correctly, leading to unexpected failures or bypasses.
Expert tips
Consider using a dedicated, well-tested DNS resolution library to handle network complexities and timeouts reliably.
Build comprehensive logging for SPF evaluations to help diagnose issues in real-time and provide clear feedback on why an SPF check resulted in a specific outcome.
Stay updated with changes in mail server behavior and new RFCs, as the email authentication landscape evolves.
Expert view
Expert from Email Geeks says that building an SPF checking library is far more complex than it appears, citing weird corner cases, macros, and PTR records as particular challenges.
2020-04-10 - Email Geeks
Marketer view
Marketer from Email Geeks mentions that SPF macros are a primary reason they avoid delving into SPF implementation, highlighting their inherent difficulty.
