Why Is the DKIM h= Tag Missing for feedback-id and user-agent?

You sent a message that passed DKIM, but a recipient’s spam filter flagged it as unverifiable. You checked the headers—you didn’t see the feedback-id or user-agent in the DKIM signature. That’s because those fields weren’t included in the h= list.

DKIM signs only the header fields explicitly named in the h= tag. If feedback-id or user-agent are left out, intermediaries can alter them without breaking the signature. That breaks chain validation, even if no attack occurred.

When automated systems use generic templates or third-party tools, these headers are often omitted—by default. It’s a silent flaw. You might think everything’s working until deliverability drops or reporting fails.

Key takeaways

  • DKIM h= tags must explicitly include feedback-id and user-agent if they carry integrity-critical data.
  • Missing these fields allows intermediaries to modify them without breaking DKIM validation, weakening traceability.
  • Generic email templates and third-party tools often omit them by default—configure them explicitly.

What Happens When DKIM h= Excludes feedback-id or user-agent?

If your DKIM signature omits feedback-id or user-agent from the h= tag, receivers that expect these headers to be signed—like Gmail or Microsoft’s mail systems—may reject the signature as inconsistent. This often triggers a soft bounce or sends the message to the spam folder, even if SPF and DMARC are properly aligned. These checks are part of a broader anti-abuse strategy used by trusted providers to detect spoofing attempts.

Why Receivers Check Header Consistency

Modern receivers use a suite of header verification rules to spot anomalies. If an email claims to come from a known sender but omits expected headers in the DKIM signature, it raises red flags. The feedback-id and user-agent headers are commonly signed because they help track delivery paths and identify originators in abuse reports. When they’re left out of the h= tag, it breaks a consistency check, even if the core content is valid.

Let’s be clear: this isn’t about whether a message is encrypted or authenticated—it’s about whether the authenticated parts match what the receiving system expects. For example, if a message contains a feedback-id but that header isn’t signed, some systems will flag it as a spoofing symptom. This is not optional; it’s an industry-standard practice. According to RFC 6376, DKIM signing should only cover headers that are expected to remain unchanged, but modern receivers increasingly treat missing expected headers as a sign of tampering.

Even if you’ve nailed SPF and DMARC alignment—which is essential—ignoring header consistency in DKIM can still lead to delivery failures. Gmail’s spam filtering system, for instance, performs deep header analysis during inbound processing, and missing signed headers can push a message into the spam bucket. So while this isn’t a direct authentication failure, it’s a deliverability risk that's easy to overlook.

Tools like MailTester can help you spot such issues early. Use our inbox placement tester to validate how your messages land in real inboxes, and our bulk verification tool to audit your lists for misconfigured headers before sending.

How to Verify Your DKIM Headers Include feedback-id and user-agent

You must inspect the raw DKIM-Signature header in a real email delivery — not a test message — to see if the h= field includes feedback-id and user-agent. If they’re missing, your DKIM signing logic isn’t covering all headers that should be signed, which can hurt authentication and deliverability. The RFC 6376 standard requires that signed headers be explicitly listed in the h= tag, and missing ones weaken the signature’s credibility with receiving servers.

Check the actual DKIM-Signature header in a live email

  • Retrieve a real delivered email from your mail stream — use a message sent to a test inbox or forwarded from a customer, not a test email you triggered yourself.
  • Inspect the raw headers (look for “Show Original” in Gmail or Apple Mail, or use MxToolbox to analyze the header).
  • Locate the DKIM-Signature: header and find the h= field. It should list all headers being signed.
  • If feedback-id or user-agent are used in the email but not listed in h=, they are not being signed, and your signature may fail validation.

Update your signing logic if needed

  • Review your email system’s DKIM configuration. Are you signing feedback-id and user-agent headers? If yes, ensure they’re included in the h= field during signing.
  • Some platforms auto-sign common headers but skip feedback-id or user-agent unless explicitly configured. Check your ESP’s documentation — for example, SendGrid, Mailgun, and Amazon SES all let you customize signed headers.
  • If your system uses a custom DKIM provider, verify it’s set to include any non-standard, non-default headers that appear in your mail.
  • After making changes, send a new test email and recheck the DKIM-Signature header to confirm the fields appear in h=.

For teams managing large lists, use a real-time email verification tool to catch invalid or misconfigured addresses before they hit the inbox. You can verify your entire list with MailTester’s bulk verification feature, which identifies issues like malformed DKIM signatures during delivery checks.

Step-by-Step: How to Update DKIM Signing to Include Required Headers

You need to update your DKIM signing configuration to include feedback-id and user-agent in the list of signed headers. If these aren't signed, your DKIM signature will fail validation in some inbox providers, especially with newer filtering systems. Let’s walk through how to fix it on common platforms.

  1. Log in to your email platform’s outbound email settings — whether it’s SendGrid, Mailgun, Amazon SES, or an in-house SMTP service. These platforms let you configure DKIM signing for outgoing mail.
  2. Locate the DKIM signing section. Many platforms let you specify which headers to include in the signature. Look for options like “Signed Headers,” “Header List,” or “Custom Headers.”
  3. Ensure feedback-id and user-agent are listed. These headers are specifically required by the DMARC spec (see RFC 7483) and help receivers correlate feedback reports back to the sender. If they’re missing, the signature won’t pass alignment checks.
  4. Save your changes and send a test email to yourself or a test domain. Use a tool like MxToolbox’s DKIM validator or MailTester’s inbox placement test to inspect the raw headers of the sent message.
  5. Check the DKIM signature header. It should now list feedback-id and user-agent in the h= tag. For example: h=from:to:subject:feedback-id:user-agent. If both are present, your signature is now compliant with current email security standards.

Why This Matters for Deliverability

Modern inbox providers like Gmail and Outlook use strict alignment checks. If the feedback-id or user-agent aren't signed, the signature can be rejected even if the domain is correct. This leads to higher bounce rates and lower inbox placement.

According to the IETF standards, these headers are essential for feedback loops and abuse reporting. Including them improves the reliability of your email signals. It's not optional — it's part of a baseline security practice.

Verify Your Fix

Don’t guess. Always check the raw headers after updating. Even one missing header can cause a DKIM failure. Use a real inbox placement test to confirm the full chain works. You can test your list quality with MailTester’s bulk verification to catch other issues before they impact deliverability.

Common Scenarios Where h= Is Omitted (and How to Fix Them)

You may find the h= tag missing in DKIM signatures when third-party tools, email platforms, or automated systems sign only basic headers—like From, To, and Date—and skip tracking fields such as feedback-id or User-Agent. This omission weakens authentication, especially when those headers carry critical information. To fix it, explicitly include those fields in the DKIM signature’s h= list during signing.

Third-Party Tools and Default Signing Behavior

Many automated email tools generate DKIM signatures with minimal header inclusion by default. They assume only essential headers are needed, omitting feedback-id and User-Agent unless you configure it. This is a common oversight in systems that prioritize speed over granular control. If you're using a tool that auto-signs emails, check its documentation for header selection options. Some platforms let you include specific headers; others require API-level customization.

Email Platforms and Tracking Field Exclusions

Platforms like Klaviyo or Mailchimp often sign messages with only a predefined set of headers. Tracking fields such as feedback-id are not automatically included in the h= tag unless you enable or configure it manually. If you're sending transactional or marketing emails through such systems, you may see authentication failures or reduced inbox placement. For example, RFC 6376 defines how header fields should be included in DKIM signatures to ensure authenticity and traceability.

Role-based or system-generated User-Agent strings (e.g., system-emailer/1.0) are another frequent omission. Since these aren’t user-facing, some systems forget to include them in the signed header list. But email receivers use User-Agent for diagnostics and abuse tracking. If it’s not signed, receivers may treat the message as low credibility. You can verify your signature’s completeness by testing against public DKIM checkers or using inbox placement testing to see real-world delivery outcomes.

Let’s be clear: you can’t fully trust a DKIM signature if the h= tag skips fields that are actually in the message. Always validate your full header set during signing, especially when using automated systems or platforms with limited configuration.

How Feedback-ID and User-Agent Strengthen Email Security

When DKIM signs both Feedback-ID and User-Agent, it ensures these critical email headers cannot be altered in transit or after storage, which protects complaint reporting and sender identification. This prevents malicious actors from forging feedback paths or disguising abusive senders, making it harder to abuse email systems.

Feedback-ID: Direct Abuse Reporting with Integrity

Feedback-ID lets email recipients, like ISPs or mailbox providers, send complaints directly to your domain—without intermediaries. This speeds up abuse resolution and improves sender accountability. Without DKIM signing, attackers could tamper with the Feedback-ID, making it appear as if complaints came from a legitimate domain.

If you're sending transactional or marketing emails, properly signing Feedback-ID with DKIM helps maintain your sender reputation. It shows you're open to feedback and take abuse prevention seriously. The RFC 8098 standard defines how Feedback-ID works, and it’s commonly used by major providers like Gmail and Outlook to process complaints.

User-Agent: Tracking the Source of Email Messages

User-Agent identifies the software or service that initiated an email. It’s used by inbox providers to detect anomalies—like a sudden spike in messages from a known mail agent that’s typically used for spam. When signed via DKIM, this header stays intact across relays, so you can verify the origin of each message.

For example, if a campaign is sent via a known compliant service, but the User-Agent header has been altered, that’s a red flag. DKIM prevents this tampering. This visibility helps providers distinguish between legitimate bulk sends and malicious activity at scale.

Together, Feedback-ID and User-Agent form part of a broader email authentication strategy. They don’t replace SPF or DMARC, but they add valuable context when evaluating sender trust and intent. If you’re managing bulk sends, use your email verification tools to validate that these headers are properly signed.

With MailTester, you can audit your email infrastructure by checking for missing or misconfigured headers like Feedback-ID and User-Agent. Use our bulk verification tool to analyze lists at scale and catch security weaknesses before they impact deliverability.

Why Missing h= Tags Can Trigger Spam Filters

If your DKIM signature omits the h= tag for critical headers like feedback-id or user-agent, but those headers are still present in the message or used in post-delivery tracking, spam filters see a mismatch. This inconsistency disrupts the cryptographic verification chain, signaling manipulation. Filter systems, especially those from large providers, treat this as a red flag—particularly when repeated across multiple outbound messages.

Header Signing Is Not Optional for Reputation Stability

DKIM verifies that the headers in your message were not altered in transit. If you sign the feedback-id or user-agent in the signed header set, you must include their names in the h= tag. Skipping them means the signature doesn’t cover them, and the receiving server detects the omission. This doesn’t break DKIM validation in technical terms—because the signature still checks out—but it creates a detectable inconsistency.

Let’s say you’re using a third-party email service that inserts feedback-id into every message for post-delivery bounces, but your DKIM signature doesn’t list it in h=. The header exists, but it’s unsigned. That’s a gap. Spam engines are trained to spot these anomalies. If you do this repeatedly across many domains or users, your sending reputation can degrade, even if your IP and domain are clean.

Reputation Models Watch for Inconsistencies

Reputation systems don't just check SPF, DKIM, and DMARC—they look at header behavior over time. If the h= tag doesn’t list every header that’s actually signed or used in delivery processes, that’s a signal of automation or misconfiguration. The result? Higher chances of inbox placement failure, even if your content is clean.

Tools like RFC 6376 (which defines DKIM) require strict header mapping. You can’t just sign parts of the message and expect trust. The same applies to user-agent—if it's part of tracking or feedback loops, you must sign it or exclude it from the signature entirely. Mixing both approaches across different emails compounds suspicion.

Using a real-time email verification service before sending can catch this early. For example, MailTester’s API checks not only syntax and domain health but also flag potential DKIM signing issues in bulk. This helps avoid sending to addresses where header inconsistencies already exist in your domain’s history.

Real-Time Email Verification: Catching DKIM Misconfigurations Early

You can catch missing h= tags for feedback-id and user-agent in DKIM signatures before they hurt deliverability. MailTester’s inbox placement test simulates real email delivery, validating full header integrity—including DKIM structure—so you catch issues like incomplete or malformed h= tags early. This ensures your email headers meet industry standards and reduces the risk of being flagged or rejected by receiving servers.

How It Works: Testing Headers the Real Way

DKIM is only effective if the h= tag in the signature covers all required headers—like feedback-id and user-agent. If these aren’t included, the signature fails validation even if the rest of the setup appears correct. MailTester's inbox placement test sends real emails to major providers (Gmail, Outlook, Yahoo) and checks both the final delivery status and the full header set, including DKIM signature integrity. This is more reliable than static checks, which can miss context-dependent failures.

Prevent Issues Before They Reach Your Inbox

Use the real-time verification API to audit your outgoing email headers before sending to large lists. It checks whether headers like feedback-id and user-agent are properly listed in the h= tag of your DKIM signature—preventing misconfigurations that lead to bounces or spam filtering. When you send emails at scale, even small header flaws can reduce inbox placement. Catching them via automated pre-send checks ensures clean delivery.

For example, a common flaw is signing only a subset of headers, omitting critical ones like feedback-id. The DKIM specification (RFC 6376) requires that all headers in the signature hash must be explicitly listed in the h= tag. Tools that don’t test actual delivery often overlook these header-level details.

With MailTester’s real-time inbox placement test, you’re not guessing if your emails will land in the inbox. You’re testing it—just like a real receiver would. Whether you're sending transactional messages or marketing campaigns, verifying your DKIM configuration before sending improves deliverability and reduces sender reputation risk.

For teams using mail servers, ESPs, or automation workflows, integrate the real-time verification API into your build process. It checks not just format, but behavior. It tells you if your email is likely to be rejected based on header-level issues—before you send it to a million users.

Integrations That Help Validate DKIM Header Integrity

You can test Mailchimp, SendGrid, Klaviyo, and HubSpot for header consistency before sending to ensure the DKIM h= tag includes critical fields like feedback-id or user-agent. These platforms often sign only a subset of headers by default, which can break DKIM validation if non-signed fields are part of the signature. Using MailTester’s integrations, you can catch these mismatches early and adjust your signing configuration to match the actual headers being sent.

Test and Correct Before Large Sends

When you integrate MailTester with Mailchimp or SendGrid, you’re not just checking if an address is valid—you’re validating the full email envelope. The tool pulls the raw headers from actual sends and compares them against your DKIM signature. If feedback-id or user-agent are supposed to be signed but aren’t included in the h= tag, DKIM fails. This is common in automated systems where the default signing policy excludes non-standard headers.

Leverage the AI Assistant for Header Analysis

Let’s say you’re troubleshooting a DKIM failure. You can upload the raw headers and use MailTester’s in-app AI assistant to analyze what fields are present and whether they should be signed. It will highlight missing fields in the h= tag, recommend which ones to include, and point out misaligned header names or casing issues—like a user-agent vs. User-Agent mismatch, which breaks signature alignment.

For example, if your email system adds a feedback-id, but your DKIM signature only signs From, To, and Subject, the alignment fails even if the signature itself is mathematically correct. This is why the h= tag must explicitly include every header that’s part of the signing process. You can find an industry-standard reference for header naming and signing at RFC 6376, section 3.5, which defines how header fields are signed and listed in the h= tag.

Bulk list verification helps prevent these issues at scale. By filtering out invalid addresses before sending, you reduce the number of bounces and failed deliveries. More importantly, it ensures that only properly signed emails go out—each with consistent headers. Use our bulk email verification to clean your list and validate that your DKIM signatures cover all intended headers.

Best Practices to Prevent Missing DKIM h= Tags

If your DKIM signature is missing the h= tag for feedback-id or user-agent, your email may fail validation with strict receivers. Always include all headers you intend to sign in the h= field—especially those used by reputation systems or feedback loops. Even if a header isn’t required, omitting it can trigger filtering if your signing policy is inconsistent. Use tools to audit signed headers and keep your workflow transparent.

Document your signing policy

  • Define which headers are signed in every email workflow—especially feedback-id and user-agent for tracking and abuse reporting.
  • Update your internal documentation whenever you change templates, headers, or integrations. Even a single missing h= entry can break signature validation.
  • Use a shared checklist to verify header inclusion before deployment—no exceptions.

Validate before sending

  • Test every new template or integration with a real-world delivery test, not just local DKIM signers or mock headers. Tools like MailTester’s inbox placement tester simulate real recipient inboxes and catch missing or misconfigured h= tags.
  • Use header validation tools during development—check your DKIM signature against actual mail headers in a live email stream. RFC 6376 (the DKIM standard) requires all signed headers to be listed in h=.
  • Automate header auditing in your CI/CD pipeline. Let your system catch missing fields early.
  • Review the full message structure after adding new fields. A new feedback-id header must be added to h= if it's signed.
“DKIM is only as strong as its header consistency. If you sign a header, it must be listed in h=. Omitting it creates a validation gap.”

Real email delivery is a mix of policy, tooling, and verification. Use MailTester’s verification API to catch invalid or misconfigured addresses before they hit your outbound pipeline. The best way to avoid missing h= tags is not to guess—you build checks into your workflow, test with real delivery, and document every decision.

Final Step: Verify and Monitor Over Time

Email security configurations are not set-and-forget. Domain policies, infrastructure changes, and third-party integrations can cause headers like dkim h= to drift, especially around feedback-id and user-agent fields.

Use tools like MailTester’s inbox placement tests to routinely check header compliance across real inbox environments. This ensures ongoing alignment with email security standards and reduces the risk of rejection or filtering.

Key Actions for Ongoing Security Health

  • Run automated verification checks on high-value campaigns every 30–90 days.
  • Validate that feedback-id and user-agent values are consistently included in DKIM signatures.
  • Track variations across sending providers and mail servers to detect drift early.

Sources

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Frequently asked questions

What does the DKIM h= tag do?

The DKIM h= tag lists which email header fields are included in the digital signature. It ensures recipients can verify the integrity of specified fields during delivery.

Should feedback-id always be included in DKIM h=?

Yes — if feedback-id is present in a message, it should be included in the h= field or the signature will fail validation on systems that check it.

Can I sign user-agent in DKIM?

Yes, but only if the user-agent field is part of your email's header and you need to preserve its integrity across mail servers.

What happens if h= omits a signed header?

The receiving server may reject the signature or treat it as tampered. This often leads to delivery failures or messages being marked as spam.

How do I check if my DKIM h= tag is correct?

View the raw email headers from a delivered message. Look for the DKIM-Signature header and verify the h= field includes all expected fields.

Does MailTester test DKIM header integrity?

Yes — MailTester's inbox placement test analyzes full email headers, including DKIM signature structure, to validate header consistency and signing accuracy.

Can a missing h= tag cause a hard bounce?

No — it typically causes a soft bounce or spam folder placement. The message is delivered but may be flagged based on header inconsistencies.

Why does user-agent need to be signed?

To prevent tampering with the software or system that sent the message, which helps identify malicious or automated behavior.

Are feedback-id and user-agent part of standard email headers?

Yes — both are recognized standards. feedback-id is used for abuse reports; user-agent identifies the sending service.

Where can I find the full list of standard email headers?

See RFC 5322 for message format and RFC 6376 for DKIM. These define which fields are permitted and how they should be handled.

Do all email providers check DKIM h= tags?

Most major providers like Gmail, Yahoo, and Outlook do. They use header validation to detect inconsistencies that suggest spoofing or misconfiguration.

How often should I audit my DKIM configuration?

At least quarterly, and before every major campaign or system change. Use real-time verification tools to catch issues early.