Sender Policy Framework (SPF) is a widely used email authentication method that helps prevent email spoofing by specifying which mail servers are authorized to send emails on behalf of a domain. However, SPF has a strict 10 DNS lookup limit, and exceeding this limit can lead to SPF failures.
To overcome this, SPF flattening replaces domain-based include mechanisms with direct IP addresses. While effective in reducing lookups, improper SPF flattening can disrupt email forwarding, causing authentication failures and delivery issues. In this guide, we’ll explore how to use SPF flattening correctly without breaking email forwarding.
Understanding SPF and Its Limitations
How SPF Works
SPF records are published as TXT records in the domain’s DNS and contain a list of authorized mail servers allowed to send emails for that domain. When an email is received, the recipient’s server queries the SPF record and checks whether the sending server’s IP is authorized.
A typical SPF record looks like this:
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v=spf1 include:_spf.google.com include:spf.protection.outlook.com -all
Each include: directive instructs the receiving server to fetch additional SPF records from third-party services. However, every include: lookup counts toward SPF’s 10 DNS lookup limit, beyond which SPF validation fails with a "Too many DNS lookups" error.
SPF Flattening: What It Is and How It Works
Definition of SPF Flattening
SPF flattening is a method of replacing domain-based include statements with their resolved IP addresses. Instead of multiple nested SPF lookups, the final SPF record contains a single list of IPs, reducing DNS lookups and preventing failures due to excessive queries.
Example of SPF Flattening
Before Flattening (Exceeding 10 Lookups):
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v=spf1 include:_spf.google.com include:spf.protection.outlook.com include:mailgun.org include:_spf.salesforce.com -all
Each include expands into multiple DNS lookups, quickly exceeding the limit.
After Flattening (Optimized SPF Record):
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v=spf1 ip4:192.168.1.1 ip4:192.168.2.2 ip4:192.168.3.3 -all
Here, all include directives are replaced with their corresponding IPs, reducing lookup counts.
Challenges of SPF Flattening in Email Forwarding
Email forwarding presents a major challenge for SPF authentication because when an email is forwarded, the forwarding server’s IP is not included in the original sender’s SPF record.
What Happens When SPF Meets Forwarding?
- A sender (user@example.com) sends an email to recipient@yourdomain.com.
- The recipient’s server forwards this email to another destination (finaluser@gmail.com).
- When Gmail checks SPF, it sees that the forwarding server’s IP is not in the original SPF record and fails SPF authentication.
How Flattening Makes Forwarding Worse
- Static SPF Records: Flattening replaces dynamic include lookups with static IPs, which don’t automatically update when providers change their mail server IPs.
- Higher Risk of SPF Failures: Forwarding servers are not accounted for in the SPF record, leading to more frequent SPF failures.
- Maintenance Overhead: SPF records require manual updates whenever a third-party email service updates its mail servers.
Best Practices for SPF Flattening Without Breaking Forwarding
1. Use a Dynamic SPF Flattening Service
Instead of manually flattening SPF records, use a dynamic SPF flattening service such as:
- PowerSPF (EasyDMARC)
- SPF Flattening by Dmarcian
- SPF-Tools (Open Source)
These tools generate optimized SPF records that dynamically update when third-party providers change their IP addresses.
2. Implement SPF Macros Instead of Static Flattening
Instead of replacing include directives with static IPs, use SPF macros that allow SPF policies to be more dynamic.
Example:
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v=spf1 exists:%{i}._spf.mailprovider.com -all
This allows real-time lookups without hitting the 10-lookup limit.
3. Use DKIM and DMARC to Complement SPF
Since SPF alone cannot reliably authenticate forwarded emails, DKIM (DomainKeys Identified Mail) helps by signing emails with a cryptographic key, ensuring that the message remains intact throughout forwarding.
Additionally, DMARC (Domain-based Message Authentication, Reporting & Conformance) can be configured with a relaxed policy to prevent strict SPF failures.
Recommended DMARC Policy for Forwarding-Friendly Authentication:
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v=DMARC1; p=quarantine; sp=none; fo=1; rua=mailto:reports@yourdomain.com;
This tells mail servers to monitor SPF failures but not reject forwarded emails immediately.
4. Monitor SPF and DMARC Reports Regularly
Use DMARC aggregate reports to analyze SPF authentication failures and make adjustments.
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Tools for SPF & DMARC Monitoring:
- yourDMARC
- MXToolBox SPF Analyzer
- DMARCian
If SPF failures are frequent due to forwarding, consider adjusting DMARC policies or using SPF macros.
Final Thoughts
SPF flattening is essential to optimize SPF records and stay within the 10-lookup limit, but improper implementation can break email forwarding. To avoid issues:
- Use dynamic SPF flattening services instead of static IP replacement.
- Implement SPF macros where possible to keep SPF records flexible.
- Complement SPF with DKIM and DMARC to ensure forwarded emails are authenticated.
- Regularly monitor SPF and DMARC reports to detect issues early.
By following these best practices, businesses can enhance email security while ensuring SPF authentication works effectively with email forwarding.
FAQs
1. What is SPF flattening, and why is it needed?
SPF flattening replaces include directives in an SPF record with direct IP addresses to reduce DNS lookups and avoid hitting the SPF lookup limit.
2. How does SPF flattening impact email forwarding?
Flattening can cause SPF failures during email forwarding because forwarding servers are not listed in the SPF record, leading to failed authentication checks.
3. What is the best way to handle SPF limits without breaking forwarding?
Use dynamic SPF flattening tools, SPF macros, and complement SPF with DKIM and DMARC to improve authentication.
4. Can SPF alone prevent email spoofing?
No, SPF is only one part of email authentication. DKIM and DMARC are necessary to fully protect against spoofing and phishing attacks.
5. How can I check if my SPF record is correctly configured?
Use tools like yourDMARC, MXToolBox, or DMARCian SPF Checker to test and analyze your SPF record for issues.
