SPF Flattening: When You Need It and When You Don't

You just integrated a new CRM or a customer support platform, and suddenly, your primary domain's email deliverability is in the gutter. You haven't changed your content, and your reputation is intact, but your emails are hitting spam folders or being rejected outright. You haven't been hacked; you have simply hit the 10-lookup wall.

SPF flattening is a technical workaround used to bypass the strict limit of 10 DNS lookups allowed in a single Sender Policy Framework (SPF) record. By resolving domain-based mechanisms like include or mx into a static list of raw IP addresses, you ensure that receiving mail servers can validate your authorized senders without triggering a "PermError" due to excessive complexity.

The 10-Lookup Constraint: Why It Exists

The Internet Engineering Task Force (IETF) established the 10-lookup limit in RFC 7208 to prevent resource exhaustion attacks. Without this limit, a malicious actor could create an SPF record that points to dozens of other domains, which in turn point to dozens more, effectively using mail servers to overwhelm DNS infrastructure in a distributed denial-of-service (DDoS) attack.

While this protects the internet's backbone, it creates a massive bottleneck for modern enterprises. A standard business stack—using Google Workspace for email, Zendesk for support, Salesforce for CRM, and Mailchimp for marketing—can easily exceed 10 lookups. This is because many of these services use multiple internal lookups just to resolve their own single include statement. For example, include:spf.protection.outlook.com might count as multiple lookups depending on how Microsoft structures its internal records. If you suspect you are nearing this threshold, use an SPF record checker to see your current count. For a deeper dive into the fundamentals, see our guide on what is an spf record.

How SPF Flattening Works: The Grocery List Analogy

Think of a standard SPF record as a recipe that tells a chef to "go to the store and buy whatever is on the list at the butcher shop, the bakery, and the dairy." To follow that instruction, the chef has to make three separate trips. In DNS terms, every include statement is a trip to another "store" (a separate DNS query). If the recipe requires more than 10 trips, the chef gives up and refuses to cook. This is a "PermError."

SPF flattening changes the instruction. Instead of telling the chef where to go, you do the shopping yourself. You visit the butcher, the bakery, and the dairy, write down every specific item they sell, and give the chef a single, massive list of ingredients. The chef now only has to look at one piece of paper.

In technical terms, a flattening tool takes a record containing multiple include statements and converts it into a single record composed of ip4 and ip6 addresses. Because IP addresses do not require additional DNS lookups, the receiving server can validate the sender instantly without exceeding the 10-lookup limit.

The Critical Distinction: PermError vs. TempError

When troubleshooting SPF issues, it is vital to distinguish between the two primary failure modes:

1. PermError (Permanent Error): This occurs when the SPF record is syntactically correct but violates the 10-lookup limit or contains too many mechanisms. This is a hard failure; receiving servers will often reject the email immediately. Flattening is specifically designed to solve this.
2. TempError (Temporary Error): This occurs when a DNS server is unreachable or a timeout occurs during the lookup process. Flattening can help mitigate this by reducing the number of external dependencies, but it does not solve underlying DNS connectivity issues.

The Hidden Risks of Flattening

Flattening is not a "set it and forget it" solution. It introduces a significant operational risk: stagnation.

When you use an include:mailchimp.com statement, you are delegating the management of IP addresses to that vendor. If they add a new data center and update their SPF record, your include statement automatically inherits that change. When you flatten that record into a static list of IPs, you break that link. If your email service provider changes their IP range and you are still using a flattened list from six months ago, your authorized emails will fail SPF checks because they are coming from an IP that is no longer on your list.

To use flattening safely, you must choose between two paths:

• Manual Flattening: You must implement a rigorous process to re-check and re-flatten your records every time a vendor makes a change—or at least once a month.
• Dynamic Flattening: You use a professional service that monitors your source domains and updates your flattened record in real-time.

The 512-Byte Limit and EDNS0

DNS records traditionally have a size limit of 512 bytes when sent over UDP. If you flatten a record that includes twenty different vendors, the resulting list of IP addresses might exceed this limit. While modern DNS extensions (EDNS0) allow for larger packets, some legacy network hardware still truncates records that are too long, which can break your authentication entirely.

If your flattened record is too large, you may need to split it into multiple records or, more effectively, move some services to subdomains. A flattened record that is too long is just as dangerous as a record that exceeds the 10-lookup limit.

Where SPF Flattening Falls Short

Flattening is a specialized tool, not a universal fix for deliverability. It cannot solve the following:

1. Sender Reputation: Flattening ensures your record is valid, but it won't stop you from being blacklisted if you send spam. If your domain is already on a DNSBL, a perfect SPF record will not save you.
2. Internal Logic Errors: If your original SPF record has conflicting statements (such as having both all and -all), flattening will simply carry those errors into a more complex format.
3. Lack of DMARC Alignment: SPF only verifies the "Return-Path" address. To truly protect your brand from impersonation, you must pair your SPF strategy with DKIM and a DMARC policy. You can learn how to implement this in our guide on how to set up DMARC from scratch.

When Flattening Is the Right Choice

Flattening is a solution to an architectural problem, not a general best practice. You should consider it in these specific scenarios:

Situation A: The Multi-Vendor Enterprise

If your organization uses a wide array of SaaS tools (marketing automation, HR portals, billing systems, and transactional mailers) and your lookup count is 11 or higher, flattening is often the only way to remain compliant with RFC 7208.

Situation B: High-Volume Transactional Mail

If you are building a product that sends millions of transactional emails, you cannot afford the latency or the risk of a "PermError" at the receiving gateway. A flattened record provides the fastest possible validation for the receiving server.

Situation C: Infrastructure Testing

When setting up new email testing infrastructure, developers often need to ensure that the environment mimics production as closely as possible. Using the Best-TempMail SPF flattening tool can help you quickly generate these records for your testing domains to ensure your QA environment is realistic. For more on testing environments, see our guide on best temporary email for developers.

When Flattening Is the Wrong Choice

Situation D: Single-Provider Setups

If you only use one service, such as Google Workspace or Microsoft 365, your lookup count is likely under 5. Flattening offers no benefit and only introduces the risk of your records becoming stale.

Situation E: Low Technical Overhead

If you do not have a dedicated IT team or an automated tool to monitor for IP changes, manual flattening is a recipe for a "blackout" where your email suddenly stops working because a vendor updated their infrastructure and you didn't.

The Professional Alternative: Subdomain Segmentation

The most effective way to manage SPF limits without the risks of flattening is "segmentation." Instead of trying to fit every authorized sender into the SPF record for your root domain, you delegate specific functions to subdomains.

1. Keep your primary corporate email on the root domain with a simple, low-lookup SPF record.
2. Move marketing emails to a subdomain like news.google.com (using your own domain).
3. Move transactional receipts to a subdomain like orders.microsoft.com (using your own domain).

Each of these subdomains receives its own independent 10-lookup budget. This not only solves the SPF limit problem but also protects your primary domain's reputation; if a marketing campaign triggers a spam filter, it won't impact your employees' ability to send one-on-one emails. For more on managing these technical limits, see our article on spf too many dns lookups.

How to Implement SPF Flattening Safely

If you have determined that flattening is necessary, follow these steps:

1. Audit Your Senders: Identify every service currently authorized to send email on your behalf. Remove any include statements for services you no longer use. A thorough cleanup can often bring you back under the 10-lookup limit without needing to flatten.
2. Prioritize Dynamic Services: Avoid manual copy-pasting. Use a service that provides a "Smart SPF" record. These services give you a single include to put in your DNS, and they handle the flattening and real-time updates on their end.
3. Monitor Record Size: Keep your flattened record under 512 characters if possible to avoid issues with legacy DNS resolvers. If the list is too long, move some services to subdomains.
4. Verify the Result: Before finalizing the change, use a validator to ensure the flattened record correctly represents all your authorized IPs. You can use Best-TempMail to view incoming headers and metadata in real-time, which is invaluable for verifying that your new record is passing checks across different providers.

Decision Guide: Should You Flatten?

Use Case: I have 8 lookups and am adding one more tool.

Recommendation: Do not flatten. You are still under the limit. Keep the include statement for easier maintenance.

Use Case: I have 13 lookups and my emails are being rejected with "SPF PermError."

Recommendation: Flatten immediately. This is a hard failure that will stop your emails from reaching the inbox. Use an automated tool to ensure the IPs stay current.

Use Case: I want my DNS to look "cleaner."

Recommendation: Do not flatten. A raw IP list is harder for a human to read and maintain than a list of domain names.

Use Case: I am a developer testing a new mail server.

Recommendation: Flattening is useful here to minimize external dependencies. You can use Best-TempMail tools to generate a clean record and then use their API-driven inboxes to verify the results.

Frequently Asked Questions

Does SPF flattening affect DKIM or DMARC?

No. SPF flattening only changes how authorized IPs are discovered. DKIM and DMARC remain unaffected. However, a flattened SPF record makes it more likely that SPF will pass, which in turn helps your DMARC alignment.

How often should I update a flattened SPF record?

If you are doing it manually, you should check for updates at least once a month. Major providers like Google or Microsoft don't change their IP ranges often, but smaller SaaS vendors might move infrastructure more frequently.

Will flattening help me get off an email blacklist?

No. Flattening only fixes "PermError" issues related to the 10-lookup limit. If your IP is blacklisted due to spam complaints, you must follow a separate process for how to get your domain off an email blacklist.

Can I flatten my record to include more than 100 IPs?

Technically, yes, as long as you don't exceed the DNS character limit. However, if you have that many IPs, you should investigate whether you are authorizing too many unnecessary services, as each IP represents a potential security hole.

What is the difference between a hard fail and a soft fail in a flattened record?

The difference lies in the "all" mechanism. -all (Hard Fail) tells the receiver to reject any mail not on the list. ~all (Soft Fail) tells them to accept it but mark it as suspicious. When flattening a record, many experts suggest using a Soft Fail initially to ensure you haven't missed any important IP addresses during the conversion.

Is SPF flattening a security risk?

It can be. By flattening a record, you are taking on the responsibility of ensuring those IP addresses are correct. If you accidentally include an IP address that belongs to a malicious actor, you have authorized them to send email as your domain. Always use trusted tools and verify your IP lists.

Can I use SPF flattening for subdomains?

Yes. Subdomains have their own SPF records and their own 10-lookup limits. You can flatten a subdomain's record just as you would a root domain's record.

Does flattening improve email speed?

Marginally. Because the receiving server doesn't have to perform multiple DNS lookups, the initial connection and validation phase of the SMTP transaction can be slightly faster. However, this is rarely the primary reason for flattening.