Summary
SPF flattening is primarily necessary to avoid exceeding the 10-DNS-lookup limit imposed by the SPF specification (RFC 7208). When an SPF record requires more than ten DNS queries to fully resolve, mail servers may treat emails as suspicious, leading to deliverability issues such as rejection or placement in the spam folder. This is especially common for domains using multiple email service providers (ESPs) or third-party sending services, as each 'include' mechanism in the SPF record can trigger additional lookups. Flattening consolidates these lookups into fewer, direct IP addresses or ranges, ensuring compliance and improving email authentication success.
Key findings
- Lookup limit: SPF records are limited to 10 DNS lookups, as per RFC 7208. Exceeding this limit causes authentication failures.
- Multiple services: Domains using several ESPs or third-party senders often exceed the lookup limit due to numerous 'include' mechanisms.
- Deliverability impact: Non-compliant SPF records can lead to emails being rejected or marked as spam by recipient mail servers.
- Consolidation: SPF flattening simplifies the record by replacing 'include' mechanisms with direct IP addresses or CIDR ranges.
Key considerations
- Dynamic SPF: Consider dynamic SPF flattening services that automatically manage changes to included IP addresses, avoiding manual updates.
- DMARC monitoring: Always monitor DMARC aggregate reports to understand the impact of SPF changes on email authentication.
- Unnecessary includes: Before flattening, review your existing SPF record for any unnecessary or redundant 'include' mechanisms that can be removed. For more, see when SPF flattening is needed and how to validate.
- DNS caching: Be aware that DNS caching can delay the propagation of SPF changes. This can also lead to errors such as CharacterStringTooLong. Monitor authentication results carefully after updates. See this guide on SPF flattening.
What email marketers say
Many email marketers find SPF flattening to be a necessary step, especially when managing email for domains that utilize various sending platforms. They often encounter the 10-DNS-lookup limit, which can severely impact deliverability if not addressed. While some advocate for automated flattening services, others emphasize the importance of careful review of existing SPF records to remove redundant entries before resorting to flattening, believing it can sometimes be an unnecessary expense or introduce new complexities.
Key opinions
- Necessity for limit: Marketers generally agree that SPF flattening is necessary if a domain's SPF record exceeds the 10-DNS-lookup limit.
- DMARC importance: There's a strong consensus on monitoring DMARC aggregate reports to properly assess the impact of SPF changes and inform flattening decisions.
- Unnecessary includes: Many believe that issues can often be resolved by optimizing existing SPF records and removing unneeded 'include' mechanisms.
- Service utility: While some find SPF flattening services valuable for managing complex records, others view them as potentially unneeded or costly.
Key considerations
- Record review: Always audit your SPF record for any redundant or unnecessary 'include' statements before implementing a flattening solution. Consider why some ESPs recommend SPF records.
- Cost vs. benefit: Evaluate the cost of an SPF flattening service against the potential deliverability improvements and complexity reduction.
- Tool integration: Consider how a flattening tool might integrate with your existing email infrastructure and monitoring solutions.
- Vendor claims: Be critical of claims made by SPF flattening service providers and verify their necessity against your specific sending setup. For example, why you shouldn't add MailChimp to your SPF. Learn more about SPF flattening pros and cons.
Marketer view
Email marketer from Email Geeks asks whether SPF flattening is truly necessary when using multiple services, noting a conflicting opinion about it being unnecessary or a waste of money.
Marketer view
Email marketer from Email Geeks asserts that flattening is necessary if a domain is exceeding the 10 DNS lookup limit. They also emphasize the importance of monitoring DMARC aggregate reports before using any SPF flattening solution.
What the experts say
Email deliverability experts universally agree that SPF flattening becomes essential when a domain's SPF record necessitates more than the allowed ten DNS lookups. They emphasize that while flattening can prevent authentication failures, it's crucial to understand the implications, such as the static nature of flattened records requiring frequent updates if included IP ranges change. Experts often advise a thorough audit of the existing SPF record to remove unnecessary entries before considering an automated flattening solution, stressing that proper DMARC implementation and monitoring are key to validating any SPF changes.
Key opinions
- RFC compliance: Experts stress that the 10-lookup limit in RFC 7208 is a hard requirement, and exceeding it will result in SPF failure.
- Dynamic versus static: While flattening addresses the lookup limit, experts caution about static flattened records requiring manual updates if sender IPs change, favoring dynamic solutions.
- Root cause analysis: Before flattening, experts advise identifying and removing redundant or obsolete 'include' mechanisms to simplify the SPF record.
- DMARC validation: Experts strongly recommend using DMARC aggregate reports to monitor the effectiveness of SPF flattening and identify any unforeseen deliverability impacts.
Key considerations
- IP stability: Assess the stability of IP addresses used by your email service providers; highly dynamic IPs make static flattening impractical.
- Automation tools: Consider using automated SPF flattening services for dynamic management if manual updates become too burdensome or error-prone. Learn more about best practices for SPF flatteners.
- Security implications: Be aware that flattening can obscure the actual senders behind a single 'include', potentially making it harder to identify unauthorized use if not properly managed.
- Impact on troubleshooting: Understand that a flattened record may make it more challenging to troubleshoot specific SPF authentication issues without detailed logging or a robust DMARC reporting setup. This can sometimes result in SPF TempError in DMARC reports. See SPF's limitations for email authentication.
Expert view
Expert from Email Geeks states that SPF flattening is necessary if a domain exceeds 10 DNS lookups, and emphasizes the importance of monitoring DMARC aggregate reports before implementing any SPF flattening solution.
Expert view
Expert from Email Geeks notes that in many cases, people incorrectly configure SPF records with unnecessary includes that could be omitted, leading to compliance issues.
What the documentation says
Official SPF documentation, particularly RFC 7208, establishes the crucial 10-DNS-lookup limit for SPF records. This constraint is designed to prevent excessive load on DNS servers during SPF evaluation. The documentation implies that any mechanism that leads to more than ten DNS queries during the SPF validation process will result in a 'PermError', which tells the receiving mail server to treat the SPF check as invalid. While "flattening" isn't explicitly detailed as a solution in the RFC, it is a common industry practice to circumvent this lookup limit by converting domain names into their direct IP addresses, thus reducing the number of DNS queries.
Key findings
- Strict limit: RFC 7208 specifies a strict limit of 10 DNS lookups for SPF record evaluation, excluding 'A' and 'MX' records when they resolve to an IP already checked.
- PermError result: Exceeding the lookup limit results in a 'PermError', which causes SPF authentication to fail and can negatively impact deliverability.
- Mechanism types: The documentation details various SPF mechanisms ('a', 'mx', 'ptr', 'include', 'exists', 'redirect') that can trigger DNS lookups, with 'include' being a frequent culprit for exceeding limits.
- No explicit flattening: While the RFC doesn't explicitly prescribe SPF flattening, it's an industry response to adhere to the DNS lookup constraint.
Key considerations
- Dynamic IP addresses: Documentation implicitly suggests that a flattened record with static IPs may become outdated quickly if included senders change their IP ranges.
- Record size: While not strictly a lookup limit issue, very long SPF records can exceed DNS TXT record length limits, sometimes driving the need for flattening solutions. Read more on the full form of SPF.
- Security implications: Understanding how 'include' mechanisms can chain lookups is critical to prevent malicious or unintended IP ranges from being authorized. Consider this in conjunction with a simple guide to DMARC, SPF, and DKIM.
- Regular review: Even with flattened records, regular review is necessary to ensure the authorized IP ranges remain current and accurate. See this discussion on SPF flattening.
Technical article
Documentation from Information Security Stack Exchange, quoting RFC 7208, clarifies that SPF implementations must limit the total number of mechanisms that require DNS lookups to 10 to prevent an unreasonable load on the DNS system.
Technical article
Documentation from GoDMARC Knowledge Base explains that SPF flattening's primary objective is to simplify SPF records, consolidating and optimizing them for enhanced email delivery and authentication.
Related resources
15 resources
Related pages
How to troubleshoot SPF authentication issues with multiple ESPs, subdomains, and CNAMEs?
How to resolve 'CharacterStringTooLong' error when flattening SPF records?
What are the best practices for using SPF flatteners and managing SPF records?
When is SPF flattening needed and how to validate SPF records?
How does SPF flattening affect email evaluation tools and are there alternatives?
Why your emails fail at Microsoft: the hidden SPF DNS timeout
Demystifying the SPF TempError in your DMARC reports
A simple guide to DMARC, SPF, and DKIM
What is the full form of SPF in email?
What causes SPF authentication dips in Google Postmaster Tools graphs?
