Email Verification Service Detecting Missing DNS Include Tag

Why Is a Missing DNS Include Tag Causing Email Bounces?

You sent an email to a customer. It didn’t bounce immediately. But weeks later, your open rate drops. Your deliverability score dips. You chase down logs, check sender reputation, audit your templates—nothing obvious. The real culprit? A missing or misconfigured DNS include tag in your domain’s SPF record.

SPF is the gatekeeper of email authentication. If the include tag is missing, SPF fails silently. No bounce. No alert. But the email gets flagged as untrusted by receiving servers. Over time, this erodes your sender reputation—even if your content is perfect.

MailTester detects this flaw during real-time verification. Not in theory. Not during a bulk send. In the moment. It checks the full DNS chain—like a mechanic inspecting every bolt before a car rolls out.

Key takeaways

• A missing or misconfigured DNS include tag in SPF can break email authentication without triggering a hard bounce.
• SPF failures from broken includes can degrade sender reputation over time, reducing inbox placement.
• MailTester identifies this flaw during real-time verification by validating the full DNS delegation chain.

What Is a DNS Include Tag and Why Does It Matter?

If your domain sends email through a third-party service like SendGrid or Mailchimp, your SPF record must include a reference to that service’s policy using the include tag. Without it, even valid email addresses can be blocked because mail servers see your domain as unauthorized to send on behalf of the third-party provider. This is a common cause of deliverability failures that only show up as hard bounces or low inbox placement — not invalid addresses.

How the include Tag Works in SPF

SPF (Sender Policy Framework) defines which servers are allowed to send email for your domain. The include mechanism lets you safely reference another domain’s SPF policy instead of listing every IP address manually. For example: if you use SendGrid, you must include include:_spf.sendgrid.net in your SPF record. This tells receiving servers: “Yes, SendGrid is authorized to send email on my behalf.”

Without this tag, your email appears to come from an unapproved source. Even if the address is real and the message is legitimate, mail servers may reject it outright. This is especially common with ISPs and enterprise email systems that enforce strict policy checks.

Why Missing Includes Cause Bounces

When a domain fails to include a required third-party SPF policy, receiving servers treat the message as potentially malicious. The sending server isn’t authorized, so the message is often marked with a hard bounce or delayed for inspection. There’s no "soft fail" — it’s usually an outright rejection.

These blocks aren’t detected by standard email validation, which checks only syntax and domain existence. They’re invisible until you test delivery across real provider inboxes. You can verify SPF policies using tools like MxToolbox or check the full alignment of your DNS records in the SPF specification.

Use MailTester’s inbox testing to catch these issues before you send. It shows whether your emails arrive in real inboxes — not just whether the address exists. This is how you confirm that your SPF, DKIM, and DMARC setup allows delivery, not just validation.

How MailTester Detects Missing Include Tags in Real Time

When you verify an email address, MailTester doesn’t just check if it’s format-valid—it scans your full SPF record in real time, following every include directive like a forensic DNS investigator. If any included domain is missing, unreachable, or misconfigured, we flag it immediately as 'risky' or 'invalid'—so you catch SPF failures before they sink your deliverability.

Real-Time SPF Chain Analysis

1. Run a full DNS lookup on the sender’s domain. This includes fetching the published SPF TXT record from the authoritative DNS server. SPF is not just a single line—it's a policy built from multiple components, and we parse the entire record.
2. Trace every include directive in the record, recursively. For example, if your record says include:_spf.google.com, we look up _spf.google.com to see what it authorizes. We don’t stop at the first level.
3. Validate each included domain independently. If an included domain doesn’t resolve to a valid DNS entry, or its SPF record fails validation, it’s treated as unreachable. This includes checking for typos, expired records, or non-existent subdomains.
4. Flag missing or broken includes with a precise verdict. If a required include is missing or unreachable, the result is marked as 'risky'—indicating high chances of failed authentication and email rejection. This helps you avoid spoofing issues that hurt sender reputation.
5. Return actionable results with full context. In our API or bulk output, you’ll see which include tag failed and why, so you can fix it quickly. No mystery—just clear, accurate detection.

Why This Matters for Deliverability

SPF failures due to missing includes are a common reason why legitimate emails land in spam or are outright blocked. According to RFC 7208, the SPF specification mandates that all included mechanisms must be reachable and valid. A single broken include breaks the chain.

Many email verification services only check basic syntax. They miss the chain reaction that happens when a nested include is misconfigured. MailTester doesn’t skip ahead. It checks every link.

Let’s say you’re using a third-party service (like a CRM or email platform) that adds an include to your SPF record. If that service’s SPF record has a typo or is no longer active, your sender policy fails—even if your own domain is set up perfectly.

You can test this safely on individual addresses with our email checker or validate your full list with bulk verification—both include full SPF diagnostics at no extra cost.

How Missing Include Tags Lead to Hidden Deliverability Risks

You might think your SPF record is fine if it passes a basic syntax check, but missing include tags can still leave your domain exposed to spam filters—even if your sending setup looks correct on the surface. Large providers like Gmail and Yahoo use deep policy analysis that detects incomplete SPF records, treating them as signs of inconsistent or untrusted sending behavior. This leads to higher spam filtering, even for legitimate mail.

Why Incomplete SPF Records Trigger Filters

SPF isn't just about syntax—it's about delegation. When you don't include all authorized sending sources, like your ESP or marketing platform, the receiver’s server sees a gap in your authorization chain. Even a small omission can trigger suspicion, especially if that source has sent mail before from a similar domain. Over time, these inconsistencies degrade your sender reputation, making your messages more likely to land in spam folders.

It’s not just about whether your SPF record is valid—it’s about whether it accurately reflects your actual sending infrastructure. Many tools only validate syntax or detect common errors like too many lookups. But they miss the difference between "valid" and "complete." A record that technically passes validation can still exclude critical senders, which means emails from those sources won’t pass SPF checks at major providers. According to RFC 7208, SPF is designed to authorize specific hosts. If parts of your chain aren’t included, the authorization is incomplete.

How MailTester Catches What Others Miss

Unlike basic validators, MailTester’s verification process examines not just SPF syntax, but its practical completeness—checking whether all necessary include tags are present and correctly delegated. It simulates how real email providers assess your domain, identifying missing links that could break deliverability even if your record appears correct at first glance. You don’t need to guess—MailTester shows you exactly where your SPF delegation is weak.

Simple syntax checks won’t catch hidden gaps. Let’s say you use SendGrid but forget to include it in your SPF. Your emails may send, but providers see no clear authorization path. Over time, this harms your reputation. MailTester helps you find these blind spots before they impact your inbox placement rates. Use the bulk verification tool to audit your entire list for issues like incomplete SPF delegation, and fix them before they hurt deliverability.

What Does the 'Risky' Verdict Mean When Include Tags Are Missing?

When an email verification service flags a domain as "risky" due to missing or malformed SPF include tags, it means the domain’s SPF record is incomplete or improperly structured, which can lead to inconsistent email delivery—especially with major ISPs. This doesn’t mean the address is invalid, but it signals that the sender’s authentication setup is flawed, increasing the chance of messages being blocked or marked as spam.

Why Missing Include Tags Matter in SPF Records

SPF (Sender Policy Framework) is a DNS-based email authentication method that tells receiving servers which IPs are allowed to send email on behalf of a domain. It relies on correct syntax—especially with include: tags, which reference third-party services like SendGrid, Mailchimp, or Amazon SES.

If an include tag is missing or misformatted—like include:example.com instead of include:_spf.example.com—the SPF check fails. This can cause deliverability issues even if the email address itself is valid. According to the IETF, SPF requires strict syntax rules, and violations are treated as failures by most modern mail servers.

What a 'Risky' Status Actually Means

A 'risky' verdict isn’t a false positive. It’s not the same as "invalid" (which means the address doesn’t exist) or "catch-all" (where all addresses on the domain are accepted). Instead, it’s a signal that the domain’s SPF configuration is weak or broken, which can harm sender reputation over time.

Mail servers may still accept mail from domains with a risky SPF record, but they often apply stricter scrutiny—especially at scale. This can lead to inconsistent inbox placement, particularly with Gmail, Apple Mail, or corporate email gateways that enforce strict policy checks. For example, messages from senders with malformed SPF are more likely to end up in spam folders or be throttled.

Let’s say you’re sending transactional emails from a customer-facing service: if your SPF record is missing include tags for your email provider, you risk losing visibility—even if the recipient is real.

Use a real-time email verification API or bulk verification tool to catch these issues before sending. MailTester identifies missing or incorrect include tags in SPF records during validation, giving you a clear picture of which domains are at risk—so you can fix configuration issues and improve inbox placement.

How to Verify SPF Policies in Your Email Verification Workflow

You can catch missing include tags in SPF records by using MailTester’s API or bulk verification to scan your list, then review 'risky' results—often pointing to incomplete SPF configurations—and use the in-app AI assistant to suggest fixes like adding missing include directives. Once applied, re-validate to confirm the policy is properly structured.

Check for Incomplete SPF Records with Real-Time Tools

1. Integrate MailTester’s real-time verification API into your sending workflow or run a bulk verification on your email list. The service checks DNS records—including SPF—for validity, including the presence of required include tags that reference third-party providers (e.g., SendGrid, AWS, Google Workspace).
2. Review the 'risky' verdicts returned by MailTester. These flags often point to SPF policies that lack proper delegation, especially when a domain uses external services but fails to include their SPF mechanisms via include tags.
3. Let the in-app AI assistant analyze flagged domains. It will highlight missing includes, suggest correct syntax, and reference common patterns used in industry-standard SPF configurations. For example, it may recommend adding include:_spf.google.com for Gmail-based systems.
4. Apply the suggested changes directly in your DNS provider’s interface. Ensure the final SPF record does not exceed 10 mechanisms (a hard limit defined in RFC 7208) and includes only valid, authorized sources.
5. Re-run verification on the same list or domain after DNS changes. MailTester confirms whether the SPF policy now passes validation and prevents deliverability issues caused by incomplete or invalid records.

Why This Matters for Deliverability

SPF isn’t just about authentication—it’s a gatekeeper. Incomplete policies trigger fails, even if the email address is valid. According to Google’s guidelines, emails from domains with misconfigured SPF are more likely to be flagged or rejected.

Use this process as a proactive step in your email hygiene routine. It stops bounces, reduces spam complaints, and improves sender reputation before you send. With MailTester, every fix is measurable. You don’t need to guess if you’ve resolved the issue—just verify again.

The Real-World Impact: What Happens When You Skip SPF Validation?

Skipping SPF validation means your emails risk being rejected or flagged as spam—especially when DNS delegation is broken and include tags are missing. Without proper SPF alignment, receivers can't verify your sender identity, leading to higher bounce rates and degraded sender reputation over time. Even a single misconfigured include tag in your DNS can break the chain and cause legitimate messages to fail silently.

How Missing 'include' Tags Tank Deliverability

Let’s say you send a campaign to 100,000 addresses using a domain with a flawed SPF record missing an 'include' tag. That single missing directive might result in a 10–15% delivery failure rate, even if all the email addresses are valid. Recipient servers see the SPF check as invalid, and many treat this as a red flag—even if your content is clean. This isn’t hypothetical: according to RFC 7208, SPF is designed to enforce sender identity, and missing includes break that chain.

When SPF fails consistently, especially at scale, email providers start treating your domain as unreliable. If even a small percentage of your emails originate from unverified sources—due to a broken include or outdated SPF—this can trigger automatic spam filtering. Some ISPs may even blacklist the domain if abuse is detected from associated IPs, regardless of intent. The damage isn’t always immediate, but it compounds with every failed delivery.

The Hidden Cost: Reputation Damage That Outlasts Fixes

Even after fixing the missing 'include' tag, the harm persists. ISPs and gateways track historical behavior. A track record of SPF failures—especially if tied to a high volume of undelivered messages—can lower your domain’s trust score. Cleaning up DNS records doesn’t erase past behavior in their algorithms. You’re not just fixing a technical flaw; you’re rehabilitating credibility.

That’s why catching SPF problems early matters. Tools like MailTester can verify your DNS records and flag include issues before they impact your sends. Use the bulk verification tool to audit your list and check SPF health across domains in a single run. You don’t need to wait for bounces to see the damage.

Remember: SPF isn’t just a technical checkbox. It’s a signal to receivers about your intent and consistency. If your domain skips validation—whether due to a missing include tag or oversight—you weaken your entire delivery stack. Stay proactive. Fix the root cause, not just the symptoms.

Why Other Services Miss This — And MailTester Doesn’t

You’re using an email verification service, but it still lets bad addresses through because it only checks basic syntax or common bounce signals — not the full SPF policy chain. Most tools stop at the end-domain SPF record, missing missing include tags in DNS delegation chains that break sender reputation. MailTester doesn’t just check if SPF exists; it maps the full delegation path in real time, catching misconfigurations that others miss.

What Most Services Skip

• They only validate SPF syntax at the end domain — not the chain of delegation.
• They don’t analyze the include tag resolution path across subdomains and third-party providers.
• They treat SPF as a static record instead of a dynamic policy chain.
• They don’t detect broken delegation paths where include tags point to non-existent or unreachable domains.
• They often fail to flag missing or malformed include tags that cause policy failures in mail servers.

How MailTester Checks the Full Chain

• It performs real-time DNS policy analysis across the complete delegation structure.
• It traces every include tag back through the DNS hierarchy to verify existence and reachability.
• It recognizes when a domain in the chain doesn’t publish a valid SPF record — a common source of delivery issues.
• It flags domains where delegation is incomplete, even if the end record passes basic syntax checks.
• This level of depth is baked into its 98.9% accuracy, not an optional add-on.

For example, if your sender domain includes include:_spf.google.com, MailTester doesn’t just accept that trust. It verifies that Google’s SPF record is active, correctly formatted, and reachable via DNS. Many tools treat this as “valid” even if the include target is unreachable.

Real-world SPF misconfigurations are common — even among major brands. According to RFC 7208, the standard for SPF, the entire policy chain must be consistent. A broken include tag breaks the chain, leading to soft bounces or rejection.

Use MailTester’s bulk verification to cleanse entire lists before sending, or check individual addresses with the email checker. Both processes include full SPF chain analysis — not just the end result, but the logic that leads to it.

How to Fix a Missing Include Tag in Your SPF Record

If your SPF record is missing an include tag for a third-party service like Google or SendGrid, emails from that service may fail authentication and land in spam. You must update your DNS TXT record to include the correct domain (e.g., include:_spf.google.com), save the change, and wait up to 48 hours for propagation. Verify the fix with a tool like MailTester to confirm SPF is now properly configured.

Step-by-Step: Fixing the Missing Include Tag

1. Log in to your DNS provider’s control panel — this could be Cloudflare, Namecheap, AWS Route 53, or another managed DNS service. Access to your domain’s DNS settings is required to edit the SPF record.
2. Locate your SPF TXT record — look for a record starting with v=spf1. It’s often the only TXT record with that prefix. Multiple SPF records can break authentication; only one should exist.
3. Add the missing include tag — if you use Google Workspace, include include:_spf.google.com. For SendGrid, use include:sendgrid.net. Double-check spelling, spacing, and syntax. Incorrect tags can break SPF altogether.
4. Save the changes — confirm the update is saved. DNS changes propagate globally, so the fix may take up to 48 hours to take effect across all mail servers.
5. Verify the corrected SPF record — after propagation, run a test using an email verification service. MailTester’s DNS and SPF validation checks confirm that your record now includes all necessary domains.

Why This Matters for Deliverability

SPF is a core email authentication method. If an SPF record is incomplete or missing required includes, receiving servers may treat your messages as suspicious or unauthorized. According to RFC 7208, SPF validation is a key step in determining whether a message should be placed in the inbox or rejected.

Even small errors — like a typo in include:_spf.google.com — can cause consistent failures. MailTester detects these issues during its inbox placement and deliverability checks, helping you ensure your outbound emails are trusted by major providers.

If you’re managing a large volume of emails, use MailTester’s bulk email verification to check entire lists for SPF- and DNS-related red flags before sending, reducing bounces and protecting sender reputation.

How MailTester’s Accuracy Ensures You Don’t Over-Flag Valid Addresses

MailTester’s 98.9% accuracy isn’t based on rules-of-thumb or outdated assumptions—it tests actual email delivery paths using live SMTP sessions. This means real domains with complex DNS setups, like those with missing include tags in delegation, aren’t wrongly flagged as invalid. You keep valid addresses, reduce false positives, and improve inbox placement without losing legitimate contacts.

Real-World Verification, Not Heuristic Guesswork

Many email verification services rely on static rules—like checking for a missing DNS tag and assuming the address is bad. That's a trap. A domain might have a missing include tag in its SPF record, but still be fully functional because it uses a valid, included policy elsewhere in its chain. Let’s say your list has a valid user at [email protected]. The SPF record might not include a subdomain, but the domain still accepts mail. Heuristic tools miss this nuance.

MailTester doesn’t guess. It validates through actual SMTP handshakes. Each address is tested as if you were sending a message—checking MX records, validating the domain, and testing delivery with real infrastructure. This eliminates false flags from misinterpreted DNS structures. It’s not about parsing records; it’s about simulating what happens in the real email world.

Testing Live Paths Prevents Over-Filtering

Traditional services often mark domains as invalid if they lack a specific tag, like include, even if the domain accepts mail. That’s why some legitimate domains get blocked—because the tool doesn’t test whether mail can actually be delivered. This harms deliverability and erodes sender reputation over time.

By using real-time SMTP sessions, MailTester detects not just whether DNS records are present, but whether an address is actually operational. This is how you avoid over-flagging addresses in domains with non-standard but valid configs. The system learns from patterns in actual delivery and adjusts for complexity, not assumptions.

For example, a company might use a third-party service with a delegated SPF that doesn’t include every subdomain. MailTester doesn’t penalize that setup. Instead, it checks whether the service’s infrastructure accepts mail—whether a message reaches the inbox. You can test this yourself before sending: verify single addresses or bulk-verify your list.

For deeper insight, you can explore how email delivery works at scale through industry standards: SPF (RFC 7208) and DKIM (RFC 6376) set the foundation, but real validation requires live checks. A system that respects these standards without over-relying on syntax checks is built to last. With MailTester, you maintain inbox placement and sender reputation by keeping your list clean—without losing good leads.

Proactive Verification: Stop Bounces Before They Happen

Every email campaign starts with a list. A single invalid address or misconfigured DNS record can trigger bounces, hurt sender reputation, and reduce inbox placement. With MailTester, you verify list health and DNS integrity before sending—catching issues like missing include tags in DNS delegation that silently break deliverability.

Automate the process by integrating MailTester with Mailchimp, SendGrid, or HubSpot. Invalid, risky, or catch-all emails are filtered out at send time, reducing bounce rates and protecting sender reputation. Over time, this leads to measurable improvements in inbox placement and engagement.

Sources

• DMARC adoption among top domains surged 75% between 2023 and 2025 — from 27.2% to 47.7% — in the wake of Google and Yahoo's bulk-sender authentication requirements. — EasyDMARC 2025 DMARC Adoption Report (2025)
• Since May 5, 2025, Microsoft Outlook requires SPF, DKIM, and DMARC from domains sending 5,000+ emails per day, rejecting non-compliant mail outright at the SMTP level with error 550 5.7.515. — Microsoft Outlook requirements (via MailOver bulk-sender requirements guide) (2025)

Keep reading

• Email authentication: SPF, DKIM, DMARC, BIMI and MTA-STS (complete guide)
• How to Verify DKIM Selector Name in DNS for Email Validation
• SPF DNS Record Error Due to Invalid IP Range in all= Mechanism
• How Invalid SPF Qualifier Causes False Negative Deliverability Issues
• How to Configure DKIM for Shared Mailbox Senders with Subdomains

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

Frequently asked questions

What happens if I don’t fix a missing DNS include tag?

Emails from that domain may be blocked or routed to spam. Over time, this harms sender reputation and reduces deliverability.

Does MailTester check all SPF tags, including 'include'?

Yes. MailTester validates the entire SPF record chain, including 'include', 'a', 'mx', and 'ptr' mechanisms during verification.

Can a valid email still be rejected due to a missing include tag?

Yes. If the domain’s SPF policy is incomplete, recipients’ servers may reject the message even if the address is real.

How does MailTester prevent false positives when evaluating SPF?

It performs live DNS lookups and validates the full delegation path, not just static record syntax.

Is SPF verification part of the standard email verification process?

Not in most services. MailTester includes it as part of its core validation, which improves accuracy and deliverability insight.

How do I know if my domain’s SPF is missing an include tag?

Use MailTester to verify a sample of your list. Addresses with 'risky' verdicts often indicate SPF flaws.

Can MailTester help me fix my SPF record?

It identifies the flaw and suggests corrections using its in-app AI assistant, but DNS changes must be made manually.

What’s the difference between a 'risky' and 'invalid' verdict?

'Invalid' means the address is malformed or doesn’t exist. 'Risky' means the domain’s DNS policy is incomplete, affecting deliverability.

Why doesn’t my email service provider warn me about missing includes?

Providers often only validate the sender’s identity, not the recipient’s DNS policy. MailTester checks both directions.

Do I need to update my SPF record if I’m using a new ESP?

Yes. Your SPF record must include every domain used to send email on your behalf, like SendGrid, Mailchimp, or AWS SES.

Can a missing include tag cause a soft bounce?

Yes. Many ISPs mark messages from domains with broken SPF as temporary failures, leading to soft bounces.

How often should I verify my list for SPF issues?

Before every major campaign or list refresh. Use real-time API checks or bulk verification to stay compliant.