Email authentication is a crucial part of ensuring deliverability, and SPF (Sender Policy Framework) plays a key role in this process. However, improper SPF flattening can lead to a sudden drop in email deliverability, causing emails to land in spam folders or even get rejected.
In this article, we will explore why SPF flattening can negatively impact deliverability and provide solutions to fix these issues while maintaining compliance with authentication standards like DKIM and DMARC.
1. Understanding SPF Flattening and Its Impact
1.1 What is SPF Flattening?
SPF records often contain multiple include mechanisms that reference third-party mail providers. However, since SPF has a 10 DNS lookup limit, flattening replaces these include statements with direct IP addresses to reduce lookups.
Example before SPF flattening:
v=spf1 include:_spf.google.com include:_spf.mailgun.org include:_spf.sendgrid.net -all
After SPF flattening:
v=spf1 ip4:192.168.1.1 ip4:203.0.113.5 ip4:185.45.66.23 -all
1.2 Why Does SPF Flattening Hurt Deliverability?
While SPF flattening helps avoid the 10-lookup limit, it introduces several problems:
- IP Address Changes: Many email service providers update their IP ranges frequently, making hardcoded IPs outdated and leading to SPF failures.
- Increased DNS Record Size: Large SPF records exceed the 512-byte DNS response limit, causing resolution failures.
- Broken SPF Alignment: Flattening can inadvertently disrupt SPF alignment, leading to DMARC failures and lower email reputation.
- Email Rejection or SoftFail: If flattened records contain incorrect or outdated IPs, SPF checks will fail, reducing deliverability.
2. Diagnosing SPF Flattening-Related Issues
2.1 Checking SPF Record Validity
Use CLI tools or online checkers like MXToolbox to validate SPF records:
nslookup -type=TXT example.com
Or use a Python script:
import dns.resolver def check_spf(domain): try: answers = dns.resolver.resolve(domain, 'TXT') for rdata in answers: if 'v=spf1' in str(rdata): print(f'SPF Record for {domain}: {rdata}') except Exception as e: print(f'Error: {e}') check_spf('example.com')
2.2 Analyzing DMARC Reports
Use DMARC reports to check for SPF authentication failures:
- Identify SPF alignment issues.
- Detect mismatched IP addresses after flattening.
- Adjust SPF records based on reports.
2.3 Checking Email Headers for SPF Failures
Inspect the Received-SPF header in an email:
Received-SPF: softfail (google.com: domain of transitioning user@example.com does not designate 192.168.1.1 as permitted sender)
If SPF fails due to an outdated IP, it indicates a need for SPF record updates.
3. Fixing SPF Deliverability Issues Post-Flattening
3.1 Use Dynamic SPF Management Services
Instead of manually flattening SPF records, use services like:
- EasySPF
- SPF-Tools These services keep track of third-party IP changes and dynamically update SPF records.
3.2 Implement SPF Macros to Reduce Lookups
SPF macros dynamically resolve lookups without exceeding the DNS limit:
v=spf1 exists:%{i}.spf.example.com -all
3.3 Subdomain Delegation for SPF Management
Separate different email providers into subdomains:
v=spf1 include:_spf.sub.example.com -all
Then define SPF records for each subdomain individually.
3.4 Use Alternative Authentication Methods
SPF alone isn’t enough; strengthen authentication with:
- DKIM (DomainKeys Identified Mail) to sign messages.
- DMARC (Domain-based Message Authentication, Reporting, and Conformance) to enforce authentication policies.
- ARC (Authenticated Received Chain) to preserve authentication on forwarded emails.
4. Best Practices to Prevent SPF Deliverability Issues
- Limit the number of IPs in SPF records to avoid exceeding DNS response limits.
- Monitor SPF records regularly and update them when third-party IPs change.
- Use
~all(SoftFail) instead of-all(HardFail) if unsure about all valid IPs. - Check email headers periodically for SPF authentication failures.
- Set up DMARC monitoring to analyze email authentication performance.
5. FAQs
Q1: Should I completely avoid SPF flattening?
Not necessarily. If used correctly with dynamic SPF services, flattening can help. However, static IP hardcoding should be avoided.
Q2: Why are my emails landing in spam after SPF flattening?
Incorrect or outdated IPs in your flattened SPF record can cause SPF failures, reducing your domain’s reputation and deliverability.
Q3: How do I test if my SPF record is working?
Use tools like MXToolbox or run:
nslookup -type=TXT example.com
Q4: Does SPF affect DKIM and DMARC?
Yes. SPF failures can break DMARC alignment if DKIM is not properly configured, leading to failed email authentication.
6. Conclusion
SPF flattening, if not managed correctly, can lead to severe deliverability issues. By using dynamic SPF services, macros, subdomains, and alternative authentication methods like DKIM and DMARC, you can maintain a strong email authentication setup while ensuring optimal deliverability.
Monitoring SPF health regularly and adjusting records dynamically is key to preventing authentication failures and improving email success rates.
