Troubleshooting SPF Flattening Errors in AWS SES

When Amazon Simple Email Service (SES) rejects a message because of an SPF flattening error, the problem is usually hidden in a maze of DNS lookups. Below is a practical, step‑by‑step guide to diagnose, fix, and prevent those errors—complete with real‑world examples and a few handy tools from The Network Tools.

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What Is SPF Flattening and Why It Matters for AWS SES

SPF Basics

Sender Policy Framework (SPF) is a DNS‑based list that tells receiving mail servers which IP addresses are authorized to send mail for your domain. An SPF record looks like:

v=spf1 include:amazonses.com include:_spf.google.com -all

Each include, a, mx, or ptr triggers a DNS lookup. The RFC limits the total number of DNS mechanisms to 10 per evaluation.

Flattening Explained

Flattening replaces those mechanisms with the actual IP addresses they resolve to, producing a single‑line record that stays under the 10‑lookup ceiling:

v=spf1 ip4:203.0.113.5 ip4:198.51.100.23 ip4:192.0.2.0/24 -all

In AWS SES, a flattened SPF record is often the difference between “delivered” and “rejected” because SES checks the SPF evaluation before accepting mail.

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Common Causes of SPF Flattening Errors in SES

Symptom	Typical Root Cause
“Too many DNS lookups” error from SES	More than 10 mechanisms after includes are expanded
Stale IPs in flattened record	Record was flattened weeks ago; provider IPs have changed
Duplicate IPs	Multiple includes resolve to the same address, inflating the count
Syntax errors after flattening	Missing spaces or stray characters introduced manually

1. Exceeding DNS Lookup Limits

Even a well‑intentioned SPF record can balloon when you add third‑party services (marketing platforms, CRM, etc.). Each include may itself contain several lookups, quickly surpassing the 10‑lookup limit.

2. Stale or Duplicate Records

Flattening is a snapshot. If a provider rotates IPs (common with cloud services), the snapshot becomes outdated, causing mismatches and unnecessary lookups.

3. Misconfigured Include Mechanisms

A typo like include:amazonses.co silently fails, forcing the receiver to perform a fallback lookup that counts against the limit.

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Step‑by‑Step Debugging Process

1. Pull the Current SPF Record

Use a DNS query tool (e.g., dig txt example.com) or The Network Tools SPF checker to fetch the live record.

dig +short txt example.com

2. Count DNS Lookups

Manually tally each mechanism, or let a validator do it. The Network Tools’ SPF validator will highlight any “>10 lookups” warnings.

3. Use a Flattening Tool

If you’re over the limit, run the record through a flattening service. The Network Tools offers a free SPF flattening utility that expands includes, deduplicates IPs, and returns a ready‑to‑publish record.

4. Validate the Flattened Record

Run the new record through the same validator. Ensure:

• Total lookups ≤ 10 (flattened records should show 0 lookups)
• No syntax errors (-all at the end, spaces between mechanisms)

5. Update DNS and Test Email Delivery

Publish the new SPF TXT record in your DNS zone. After TTL expires (or force a refresh), send a test email from SES and check the SES sending logs for a “PASS” SPF result.

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Real‑World Example: Fixing a 10‑Lookup Violation

Scenario

A SaaS startup uses:

v=spf1 include:amazonses.com include:_spf.google.com include:mailgun.org include:sendgrid.net -all

SES reports “SPF check failed – too many DNS lookups”.

Solution Walkthrough

Step	Action	Result
A	Retrieve the full SPF tree using dig +trace	amazonses.com → 2 lookups, google.com → 4, mailgun.org → 3, sendgrid.net → 3
B	Total = 12 lookups (exceeds limit)
C	Run the record through The Network Tools flattening service	Output: v=spf1 ip4:54.240.0.0/18 ip4:64.233.160.0/19 ip4:198.2.128.0/17 ip4:167.89.0.0/17 -all
D	Validate the flattened record	0 lookups, syntax clean
E	Update DNS, wait 5 min, send test email	SES logs show SPF: PASS

The flattened record now fits comfortably under the limit, and email deliverability is restored.

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Best Practices to Prevent Future Errors

Keep SPF Records Lean

• Prefer IP ranges over includes when you control the sending infrastructure.
• Remove obsolete services promptly.

Automate Flattening

• Schedule a weekly job that pulls the current SPF, flattens it, and updates DNS if changes are detected.
• Use a CI/CD pipeline to version‑control your DNS TXT records.

Monitor with DNS Health Checks

• Set up alerts for SPF syntax errors or lookup count spikes.
• The Network Tools offers a DNS health monitor that can ping your SPF record daily and email you on anomalies.

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FAQs

Q: Do I need to flatten SPF for every domain that uses SES?
A: Only if the combined includes push you past 10 lookups. Small domains with a single include:amazonses.com are fine.

Q: How often should I re‑flatten my SPF record?
A: At least once a month, or whenever you add/remove a third‑party sender.

Q: Can I have both a flattened SPF and a regular one?
A: No. DNS will return the first TXT record it finds. Keep a single, authoritative SPF record.

Q: What if I exceed the 255‑character limit for a TXT record?
A: Split the record into multiple strings within the same TXT entry; most DNS providers handle this automatically.

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Bottom Line

SPF flattening errors in AWS SES are rarely mysterious—they’re usually a matter of too many lookups, stale data, or a typo. By systematically pulling the record, counting lookups, flattening with a reliable tool (like The Network Tools), and validating before you publish, you can keep your email flowing smoothly. Adopt the best‑practice checklist above, and you’ll stay ahead of SPF pitfalls before they impact your customers.