Why is SPF include tagging a critical part of email security?

You send emails through a third-party platform, but your messages get flagged or rejected. You’ve checked the headers, reviewed the logs, and still can’t find the root cause. It’s not your content. It’s not your list. It’s likely a misconfigured SPF record — and more specifically, an improperly used include tag.

SPF is a foundational email authentication method that tells receiving servers which IP addresses are allowed to send mail for your domain. Without it, your emails risk being filtered as spam. The include tag lets you securely delegate sending authority to services like SendGrid, Mailchimp, or AWS SES — eliminating the need to manually manage every IP address. But when it’s used incorrectly, it can break authentication, reduce deliverability, and harm your sender reputation.

Key takeaways

  • Improper include tags in SPF records are a leading cause of email authentication failures.
  • Use include only for trusted third-party services with verifiable SPF configurations.
  • Excessive or nested includes increase the risk of exceeding SPF's 10 DNS lookup limit.

What does the SPF include tag actually do in the DNS record?

The SPF include tag allows your domain to authorize specific third-party services to send email on your behalf by referencing their SPF record in your own DNS configuration. This means if you use a marketing platform or cloud service, you don’t need to list every IP address they might use — you simply include their SPF policy, and their authorized sending IPs are automatically trusted for your domain. It’s efficient, secure, and reduces the risk of missing an IP when a sender adds servers.

How inclusion works in practice

Let’s say you run a business and use SendGrid for transactional emails. Instead of listing SendGrid’s 50+ IPs directly in your SPF record — which would be error-prone and difficult to maintain — you use include:sendgrid.net. That single line tells receiving mail servers: “Any IP authorized by SendGrid’s SPF policy is also authorized to send from my domain.” You can add similar tags for your CRM, email newsletter service, and more.

This delegation model is standard in email infrastructure. The IETF’s RFC 7208, the official SPF specification, explicitly supports include as a way to manage complex, multi-sender environments without duplicating policy details.

Why it reduces risk and simplifies maintenance

When a sender like Mailchimp or Stripe updates their infrastructure — adding new servers or deprecating old ones — you don’t have to manually update your SPF record. The include tag lets you rely on their SPF record, which they control and update regularly. This reduces the chance of a forgotten IP or an expired IP causing a send failure.

But here’s the catch: include chains can get long, and exceeding the 10 DNS lookup limit in SPF can break your policy. Always audit how many include tags you’re using. Tools like MxToolbox or the RFC 7208 document can help validate your setup.

Before relying on include tags, verify your SPF setup using an inbox placement tester or a real-time verification API. You can test how your SPF record behaves with tools designed to simulate real inbox delivery and check for common errors. Test your email deliverability before sending to ensure your inclusion chain isn’t breaking the chain of trust.

What are the most common mistakes with SPF include tags?

Using SPF include tags incorrectly can break email authentication, trigger spam filters, or block legitimate messages. Common issues include referencing domains without SPF records, exceeding the 10 DNS lookup limit, or misaligning SPF with DMARC policies. These errors aren’t just theoretical—many senders see hard bounces or deliverability drops due to them. Let’s break down the three biggest pitfalls and how to avoid them.

Missing SPF records on referenced domains

  • Don’t use include for a domain that doesn’t have a published SPF record. The SPF validation process fails if the DNS lookup resolves to no record, causing the entire SPF result to become neutral or fail.
  • Always confirm the target domain has an SPF record before including it. You can check this using MxToolbox or a DNS lookup tool before deploying a new SPF policy.
  • Using include without proper validation is like building a bridge to a nonexistent island—it doesn’t just fail, it breaks trust in your entire email chain.

Exceeding the DNS lookup limit

  • Each include tag counts as one DNS lookup. With a limit of 10, exceeding it (for example, by including 7 third-party vendors) leads to SPF failure, even if all domains have valid records.
  • Be conservative. Prefer include only for providers you absolutely need to authorize, and avoid nesting includes (e.g., include:provider1.com include:provider2.com) if not required.
  • Consider using a single, verified proxy or forwarder domain to consolidate includes, if applicable—but only if the setup is audited for alignment and compliance.

Ignoring DMARC alignment

  • SPF can pass, but DMARC can still fail if the domain in the From header doesn’t match the domain used in the SPF authentication (i.e., sender vs. from alignment).
  • Using include tags for third-party senders (like your CRM) without ensuring that the authenticated domain matches your From domain is a common misstep.
  • Use MailTester’s email checker to verify whether a recipient address will authenticate correctly before sending, and test full authentication chains using inbox placement testing to catch alignment issues early.

How to safely use SPF include tags without breaking deliverability

You can safely use SPF include tags by limiting them to only trusted, well-maintained services with published SPF records, using domain-specific includes like include:spf.mailchimp.com instead of broad ones, and ensuring total DNS lookups stay under 10 by combining mechanisms like all:~all only after validating your final count. This prevents SPF failures that lead to bounces or spam filtering.

Use only verified, specific includes

Don’t use generic or broad include tags like include:third-party.net—they’re high-risk and often point to services with no reliable SPF setup. Instead, include only domains you’ve confirmed through their official documentation. For example, Mailchimp, SendGrid, and HubSpot publish their SPF records publicly, so you can reference them directly with confidence.

Let’s say you send transactional emails via SendGrid. Their official documentation confirms the correct include is include:sendgrid.net. You can trust that one because SendGrid maintains it, and it’s listed in their public API guides. Using include:sendgrid.net is secure; using something like include:smtp.service is not.

Always verify that the domain in the include tag has a published SPF record. You can check this using tools like MXToolbox or RFC 7208, which defines how SPF is structured and validated.

Watch DNS lookup limits and test thoroughly

Each include tag counts as one DNS lookup. SPF allows a maximum of 10 lookups per policy. If you exceed this, your SPF record fails, and your mail may be rejected by receiving mail servers.

If you’re using multiple third-party services, group them smartly. For example, if you use both Mailchimp and Klaviyo, use include:spf.mailchimp.com and include:spf.klaviyo.com—not separate third-party fallbacks. Then, validate your total lookup count using a DNS lookup checker like RIPE’s DANE tool or a dedicated SPF validator.

Once you confirm the total is under 10, you can safely apply all:~all to indicate soft fail (preferred over all:-all for deliverability). Test your setup with a real email checker before going live. You can spot issues early with an email verification tool that checks for deliverability blockers like malformed headers or failed DNS lookups.

What happens when too many includes break SPF validation?

When an SPF record requires more than 10 DNS lookups during evaluation, receivers reject it outright. The SPF specification limits lookups to 10; exceeding this triggers a temporary failure. Emails from such domains are often marked as unauthenticated, and major platforms like Gmail, Yahoo, and Outlook treat them as potential spoofing attempts, drastically reducing inbox placement.

Why the 10-lookup limit matters

SPF checks are recursive: each include directive forces a DNS query. If you chain multiple includes — say, for a marketing team, a third-party email service, and internal tools — the count adds up fast. Once you hit 11 or more lookups, the validation fails before it completes. The receiver doesn’t even see the rest of the policy.

This isn’t just a technical quirk. It’s a security control. The 10-lookup limit, defined in RFC 7208, prevents abuse and DNS amplification attacks. But it also means overly complex SPF records become a deliverability minefield.

Consequences for your email program

When SPF fails, your messages don’t get through — or land in spam folders. Gmail and Yahoo apply strict validation, often blocking delivery entirely if SPF is invalid. Even if the message reaches the inbox, the sender reputation suffers, especially if it happens consistently across multiple sends.

Let’s be clear: it’s not just about one bounced email. It’s about trust. A failed SPF check signals to receivers that your infrastructure isn’t properly configured, increasing the likelihood of being flagged as a source of phishing or spam.

Tools like MailTester’s Email Checker can verify whether an address’s domain has a valid SPF record before sending — helping you catch broken records or overly complex setups in advance.

Some services, such as SendGrid or Mailchimp, require you to include your own SPF mechanism when sending via their platform. That can add up quickly. If you’re using multiple third parties, don’t assume they’ll handle SPF properly. Many do not.

Even if your SPF record appears to pass a quick check, it may still fail under real-world conditions due to too many includes. The only way to ensure you stay under the limit is to audit your record against known providers and avoid over-delegation.

For ongoing email infrastructure health, consider using MailTester’s integrations with platforms like Mailchimp or HubSpot to validate SPF policies in bulk — especially during onboarding or when adjusting sending behavior.

A well-structured SPF policy does one thing: it clearly authorizes legitimate sending sources. Avoid overloading it. Keep it lean. Use include only when necessary — and always monitor DNS lookup counts.

You can audit your SPF record for include-related risks by checking every include directive to ensure the referenced domains resolve to valid SPF records, count total DNS lookups to stay under the 10-lookup limit, and confirm those domains are still active and correctly configured. Neglecting any of these steps opens your email to deliverability failures or spoofing attempts.

  1. Use a DNS lookup tool to resolve every include directive. Tools like MXToolbox or DNSStuff can trace the full chain of SPF lookups. You’re not just checking your own record — you’re verifying every domain your SPF references is still live and publishing an SPF record. If an included domain has changed its configuration or deleted its SPF entry, your record breaks.
  2. Count all DNS lookups involved in your SPF chain. Each include, redirect, and mechanism (like ip4 or mx) counts toward the 10-lookup limit defined in RFC 7208. Exceeding this limit causes your SPF check to fail. For example, include:example.com counts as one lookup. If example.com itself includes another domain, that’s a second. Do this recursively across all chains.
  3. Verify that all included domains actively publish valid SPF records. Not all domains you include will maintain SPF records. Some drop them, switch providers, or misconfigure them. An outdated or missing SPF record in a chain causes the entire validation to fail. Use a tool like DNSDumpster to inspect historical records if active ones aren’t visible.
  4. Remove or replace references to domains that no longer exist or are inactive. If a third-party service you’ve included (like a marketing platform or email gateway) changes its domain or drops SPF, your sender reputation can be damaged. Replace outdated includes with current, verified ones or remove them if no longer needed. This is especially critical for partners with poor SPF hygiene.
  5. Test your final SPF configuration using a real-world test. After updating, run your record through an inbox placement tester — like MailTester’s inbox placement test — to confirm the SPF check passes from the perspective of receiving servers like Gmail or Outlook. Real inbox testing is the only way to confirm your changes improved deliverability.

When in doubt, use MailTester’s bulk list verification

If you’re managing a large list of senders or domains, manual auditing is slow and error-prone. Use MailTester’s bulk email verification to validate SPF and other factors across thousands of addresses at once. It checks not just syntax but real-world deliverability risks tied to infrastructure.

Can you use SPF include with multiple third-party senders?

You can use the SPF include tag with multiple third-party senders, but each additional include increases the risk of exceeding the DNS lookup limit—typically capped at 10 lookups per SPF check. Exceeding this limit causes your SPF record to fail, which harms deliverability. Instead of stacking includes, consider delegating through a single, well-managed sender domain or aligning DMARC policies with DKIM and SPF to reduce reliance on overly complex SPF records. This approach improves consistency and avoids common configuration traps.

Why stacking includes backfires

Every include tag triggers a DNS query to retrieve the referenced SPF record. If your setup includes multiple vendors—like your CRM, email service provider, and newsletter platform—each lookup counts toward the 10-lookup limit defined in RFC 7208. If one of the included records has its own include tags, you're quickly consuming lookups just to resolve the chain. This isn’t theoretical: a single misconfigured chain can push a record past the limit, resulting in a "PermError" in SPF validation.

Alternative approaches to secure delegation

Instead of relying on multiple includes, you can consolidate senders under a single domain configured for outbound email. This domain’s SPF record can then include the required third parties once, reducing the total number of lookups. For example, if you use SendGrid, Mailchimp, and a helpdesk tool, you could have a shared domain like senders.yourcompany.com and set up a clean SPF record there.

Alternatively, focus on aligning your DMARC policy with DKIM and SPF. By using DKIM signatures—especially with selector-based validation—you can delegate trust without relying on SPF includes. DMARC allows you to specify that only emails with valid DKIM or SPF (or both) from authorized domains should be allowed to send on your behalf. This flexibility lets you scale without overloading SPF. The IETF outlines this approach in RFC 7483, which describes DMARC alignment as a core part of email authentication.

For a real-world test of your domain’s SPF, DKIM, and DMARC setup—including inbox placement—try our inbox-placement test, which checks actual deliverability in Gmail, Outlook, and other major providers without sending a single message.

How do DMARC and SPF work together with include tags?

DMARC relies on SPF and DKIM to validate email authenticity. If your SPF record uses an include tag that misaligns with your DMARC policy—like including a third-party domain that doesn’t match your sender domain—DMARC will treat it as a failure, even if the message is legitimate. This can lead to delivery rejection, especially if DMARC is set to reject:

SPF includes and DMARC alignment: a critical match

DMARC enforces alignment between the from header domain and the domain used in SPF authentication. When you use include to delegate SPF validation to a third party (like a marketing platform or email service), that third party’s domain must align with your own in the From header. If it doesn’t—say, a include points to sendgrid.net, but your message’s From header says yourcompany.com—DMARC fails.

If DMARC policy is set to reject, and SPF fails due to a misaligned include, your email gets blocked—even if you’re a legitimate sender. This is a common cause of false positives in email deliverability. The fix isn’t disabling includes; it’s ensuring your include tags align with your actual sending domains.

Let’s say you send via Mailchimp. You should include include:_spf.mailchimp.com in your SPF record, but only if your message's From header matches the domain you’ve authorized in Mailchimp’s setup, like newsletter.yourcompany.com. Otherwise, DMARC sees a mismatch and enforces the policy.

Always align SPF includes with your DMARC policy. If you’re confident about your sends and want strong protection, set DMARC to reject. But only after auditing your SPF includes to ensure they properly reflect your sending practices. For less aggressive environments, quarantine or none may be safer during transition.

For real-time validation of SPF and DKIM records, you can test your configuration using a tool like MailTester’s email checker, which validates syntax and alignment. You can also verify entire lists for deliverability risks before sending.

SPF’s structure is defined in RFC 7208, and DMARC's behavior is documented in RFC 7483. These standards enforce consistency—misalignment is a technical violation, not a policy loophole.

How to verify SPF configurations in real-time before sending

Use MailTester’s real-time verification API to test individual or bulk email addresses with full SPF checking—ensuring the sending domain’s SPF record explicitly allows the outbound IP, including validating any include tags. This catches issues like missing or invalid includes before you send, reducing bounce rates and protecting sender reputation. Simulate delivery across real inboxes with the inbox-placement test to catch SPF-related delivery failures early. You can also check if a single address is valid before sending via our email checker.

Test SPF validity at scale with real-time API checks

When you’re building an email campaign or setting up a new sending infrastructure, don’t guess whether your SPF record is correctly delegating permissions. Instead, integrate MailTester’s verification API into your sending workflow. It checks whether the sending domain’s SPF record authorizes the outbound IP and validates any include tags—like include:spf.example.com—to ensure the delegation is legally sound and does not introduce unintended access.

For example, if you use a third-party email service provider (ESP), you need to ensure their SPF record is properly included. A malformed or incorrect include tag can cause legitimate emails to fail SPF checks. Our API detects those issues instantly, so you know whether the configuration will hold up under real-world validation. This is particularly critical when using shared IP pools or sending from multiple domains.

Simulate email delivery to catch SPF blockages before they happen

Beyond individual address checks, send your messages through MailTester’s inbox-placement tester to see how they perform in real inboxes across major providers. This test mimics how an email would be processed when sent from your setup, revealing whether SPF mismatches are causing rejections—even if your DNS looks correct.

SPF validation fails silently in many cases. An email might pass DNS checks but still be blocked if the sending IP isn’t authorized by the domain’s SPF record. Our inbox-placement test surfaces these failures by simulating actual delivery paths. It checks for SPF, DKIM, DMARC, and other authentication protocols, so you don’t rely on guesswork.

SPF is a critical layer of email security; as outlined in RFC 7208, it defines which hosts are permitted to send on behalf of a domain. However, misconfigurations are common, especially with include tags. Use this layer of testing not as a one-off, but as part of your send validation process—just like you’d verify a URL or check a login token.

What should you monitor after implementing SPF includes?

After adding SPF includes, monitor sender reputation, bounce rates, and recipient list health. A sudden rise in bounces often means SPF validation is failing. Use verified tools to spot issues early and clean your list before sends. This keeps your inbox placement stable and avoids being flagged as malicious.

Verify sender reputation and domain health

  • Check your domain’s reputation using Spamhaus or MxToolbox — both provide real-time data on blacklists and reputation scores.
  • Review Microsoft’s Sender Score (available at sender-score.microsoft.com) to track how mailbox providers perceive your sending behavior.
  • Look for sudden drops in reputation after SPF changes; they can signal misconfigurations or unexpected policy enforcement.

Track delivery performance and list quality

  • Monitor bounce rates closely. A spike after SPF deployment often means a failing include or over-strict policy.
  • Use MailTester’s bulk list verification to scan your entire sender list, flagging addresses with invalid or weak SPF context before you send.
  • Validate individual addresses right before sending with the email checker to catch edge cases in real time.
  • If you send at scale, run inbox placement tests with MailTester’s inbox tester to verify whether your messages land in inboxes versus spam.
Even a single incorrectly included domain in SPF can trigger a hard failure for entire mailstreams. Verification prevents that.

Remember: SPF includes are not a one-time setup. As you grow or onboard new services, revisit the list. Use your verified data to confirm that every included domain is legitimate and properly aligned with your sending policy.

How does MailTester help you build secure, future-proof SPF structures?

SPF structures can break silently when include tags reference domains that change or drop their records. MailTester’s real-time API validates SPF legitimacy at the moment of delivery, not just during setup, ensuring your infrastructure holds under real-world conditions.

Bulk list verification identifies domains with broken include chains, missing SPF records, or misconfigured policies—issues that lead to authentication failures and deliverability drops. These insights let you act before they impact your sender reputation.

With a verified accuracy rate of 98.9%, MailTester delivers reliable results for audits, migrations, and ongoing optimization. This precision means you can trust your data when refining your email infrastructure.

Sources

Keep reading

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

How many SPF includes can I use before causing a failure?

More than 10 DNS lookups in total—counting includes, expands, or redirects—will cause SPF to fail. Each include counts as one lookup.

What happens if a domain referenced in an SPF include has no SPF record?

SPF validation fails. The include tag cannot resolve, which breaks the chain and causes the entire record to fail.

Can SPF include tags be used for internal team email delegation?

Yes—but only if the target domain has a valid SPF record. Use sparingly and prefer internal infrastructure policies over external includes.

Do all email platforms require SPF to send?

No platform enforces SPF directly, but the absence of SPF or a broken SPF record leads to high spam filtering and low inbox placement.

Should I use include:all in my SPF record?

No. include:all is not a valid mechanism. Always reference properly published SPF records from trusted domains.

Can I replace SPF includes with DKIM instead?

DKIM covers message integrity but not sender IP authorization. Use both: SPF for IP allowance, DKIM for content signing.

How often should I audit my SPF include tags?

At least quarterly, and anytime you add, remove, or change a third-party sender. Use automated tools for recurring checks.

Can SPF include tags cause email to be marked as spam?

Not directly. But broken SPF due to malformed includes can result in authentication failure, which leads to spam filtering and rejection.

What’s the difference between SPF include and SPF redirect?

include fetches another domain’s SPF record for authorization. redirect replaces your record with another domain’s, and the behavior is not recommended.

Is it safe to use include tags for multiple marketing platforms?

It can be safe—but each additional include increases risk. Prefer consolidated platforms or use DMARC with DKIM for multi-service handling.

Can MailTester test if my SPF includes are valid during sending?

Yes. The real-time API checks SPF validity at send time, including include chain resolution, before delivery.

Do disposable email domains affect SPF include validity?

No. Disposable domains are typically excluded via list hygiene tools, not SPF. Focus on filtering them out during list cleaning.