What causes the DKIM signature expired x= tag error?

You send an email, and it bounces back with a DKIM signature expired x= tag error. Not a delivery delay. Not a spam filter. A hard failure due to a timestamp that’s already passed.

That 'x=' tag in your DKIM signature isn’t a typo — it’s a strict expiration clock. When the receiving server checks it and finds the date has passed, the message fails validation. This isn’t a glitch. It’s a design feature — but one that breaks when not managed properly.

Key takeaways

  • The 'x=' tag in a DKIM signature sets an explicit expiration time; if the email is processed after this time, validation fails.
  • Fixed timestamps in DKIM records often expire during long delivery windows, especially when sending isn’t time-stamped dynamically.
  • Misconfigured or stale DNS records for DKIM are a common root cause, especially on domains with irregular send volumes or automated, infrequent email flows.

Why does an expired DKIM signature impact deliverability?

When a DKIM signature expires, it means the cryptographic proof that an email was sent from your domain and hasn’t been tampered with is no longer valid. Mail servers can’t verify the signature, so they treat the message as untrusted—often routing it to spam or rejecting it outright. This breaks the chain of trust essential for inbox placement at Gmail, Outlook, and Yahoo.

How DKIM works in practice

DKIM signs each email with a digital fingerprint tied to your domain’s public key. Recipient servers validate that fingerprint using your domain’s DNS record. If the signature is expired, the server finds no valid proof—it’s like showing up at a door with a stale key.

Let’s say you’re sending transactional emails from a system that doesn’t refresh DKIM keys automatically. Even one expired signature in a high-volume send can trigger filters at providers like Gmail. Some systems now reject messages with any verified DKIM failure, especially if they occur repeatedly.

Consequences of repeated failures

Every failed DKIM check adds to your sender reputation risk. Major providers track this behavior across their network—you’re not just failing one message, you’re signaling inconsistent or misconfigured infrastructure.

While exact metrics are not consistently published, industry data from sources like DMARC.org and RFC 6376 confirm that missing or invalid DKIM checks are among the top reasons for email rejection. For volume senders, even one failed DKIM check can reduce inbox delivery by 10–30% over time, especially if not corrected.

Even if your email technically passes other checks (SPF, DMARC, sender reputation), a single invalid DKIM signature can pull an entire campaign into spam. This is especially true when your email volume is high—reliability becomes critical.

Prevention and proactive verification

Use tools like MailTester’s bulk verification to scan your list before sending, ensuring domains have valid, active DKIM records. You can also validate sender alignment with inbox placement testing to confirm your email reaches inboxes consistently.

Check your domain’s DKIM record monthly. If you use automation or third-party platforms, confirm they refresh keys before expiry. Tools like MailTester’s real-time API help catch issues before they impact campaigns.

Expired DKIM isn’t just a technical glitch—it’s a deliverability red flag. Fixing it early avoids lost opens, lower conversion, and long-term blacklisting risk.

How can you verify if a DKIM signature has expired?

You can verify if a DKIM signature has expired by checking the x= tag in the DKIM-Signature header. This tag contains a Unix timestamp. If the timestamp is earlier than the current time, the signature has expired. This check is critical because expired signatures fail validation, leading to delivery issues or spam filtering.

  1. Retrieve the raw email headers from a received message. Tools like Gmail’s "Show original" or third-party email analyzers can provide this. The DKIM-Signature field is embedded here, usually in the first few lines.
  2. Locate the x= tag within the DKIM-Signature field. It appears as x=1700000000, where the value is a Unix timestamp. This timestamp marks the signature's expiry time.
  3. Convert the timestamp to a readable date. Use any Unix timestamp converter (like those found at epochconverter.com or unixtimestamp.com) to see when it was set. If that date has passed, the signature has expired.
  4. Compare the expiry time to now. If the timestamp is before the current UTC time (check via a reliable source like time.gov), the signature is no longer valid.

Automating DKIM Expiry Checks at Scale

Manually checking each message is impractical for senders with high volumes. Let’s streamline this with automation.

  • Use MailTester’s real-time verification API to validate sender domains and detect expired or invalid DKIM signatures across your email list. The API returns structured results, including signature status, expiry indicators, and alignment issues.
  • For bulk checks, use MailTester’s bulk verification to process thousands of addresses and flag domains with expired DKIM or poor authentication health.
  • Combine results with an inbox placement test via MailTester’s inbox tester to see if expired signatures are impacting deliverability to Gmail, Yahoo, or Microsoft inboxes.
DKIM isn’t just about signing—validity over time matters. An expired signature is a delivery risk, even if SPF and DMARC pass.

When testing with MailTester, you get clear, actionable feedback: not just “valid” or “invalid,” but whether a signature is expired, malformed, or mismatched. This insight lets you fix domain issues before they hurt sender reputation. Integrations with platforms like SendGrid, HubSpot, and Klaviyo via MailTester’s integrations make this part of your workflow, reducing manual work and preventing bounces.

What does the 'x=' tag in DKIM actually mean?

The 'x=' tag in DKIM specifies the expiration time of the signature using Unix epoch time. If the receiving server sees that the signature has expired, it treats it as invalid—regardless of whether all other fields are correct. Modern email providers rely on this tag to prevent replay attacks and ensure time-bound signature freshness.

Why the 'x=' tag matters in practice

While the DKIM standard defines 'x=' as optional, it's now widely used by major providers like Gmail, Microsoft, and Yahoo. These systems expect signatures to be valid within a tight window—typically 24 to 72 hours. If a signature’s 'x=' value is in the past, the server ignores it entirely, even if the public key and signature match perfectly.

Let’s say you send an email at 10:00 AM UTC, and your DKIM record includes 'x=1720000000' (which corresponds to a time in the past). Even if your SPF and DMARC are correct, the receiving server will reject the signature because it’s expired. This can cause deliverability issues silently—no bounce, just an inbox drop.

How to prevent ‘x=’ expiration errors

Most email platforms auto-manage DKIM time stamps, but self-hosted or custom setups can misconfigure them. The key is to ensure your signing system sets 'x=' to a time in the future—usually within the next 24–72 hours. Double-check the epoch time you’re using; a small mistake in calculation can break delivery.

Use tools like MailTester's inbox placement tester to see how your DKIM setup performs in real inboxes. You’ll see if signatures are being rejected due to age, even if other checks pass.

The DKIM specification itself defines the 'x=' tag in RFC 6376, and while it’s optional, ignoring it risks your messages being filtered or flagged. It’s a small tag with big consequences.

You don’t need to manage 'x=' manually if you use a trusted email service provider. But if you’re running your own system—or testing DKIM headers—you must ensure 'x=' is present and correctly set. Otherwise, your email authentication is incomplete, and deliverability suffers.

How to prevent expired DKIM signatures proactively

If your DKIM signature shows an x= tag error, it means your signing key has expired. Prevent this by ensuring your email system or ESP automatically regenerates the signature with a new x= timestamp on every send. Avoid static DNS records with fixed expiration times. Instead, use rotating domain-level keys with multiple selectors and regular key cycles. Audit your DNS records monthly using tools like MxToolbox or MailTester’s inbox-placement test to catch issues early.

Key actions to avoid DKIM expiration

  • Verify your ESP or email platform auto-generates a new x= timestamp with every email send—this is fundamental. Static keys with hard-coded expiry times will fail over time.
  • Use a domain-level DKIM key with multiple selector values. This allows you to rotate signing keys without breaking prior authentication, reducing downtime risk.
  • Implement a scheduled rotation policy—rotate keys every 30 to 90 days—so expired signatures don’t accumulate. A single key used for years increases exposure if compromised.
  • Regularly audit your DNS record using a free tool like MxToolbox or run a MailTester inbox-placement test to check the current state of your DKIM configuration in real-world inboxes.
  • Monitor for unexpected failures in sending logs and bounce reports. An increase in authentication failures may point to expired or misconfigured DKIM keys.

Why this matters

Even a single expired DKIM signature can hurt your sender reputation. Many ISPs and inbox providers now reject or mark low-reputation messages as spam if they fail authentication checks. According to RFC 6376, DKIM signatures must be valid at the time of receipt. If the x= tag is expired, the signature does not pass validation.

Let’s be clear: manual key updates are unsustainable at scale. Automated rotation and real-time verification are the only reliable way to maintain consistency across large volumes. Use MailTester’s inbox placement test to simulate how your authenticated messages appear in real inboxes across providers like Gmail, Outlook, and Yahoo.

With proper key management and proactive auditing, you eliminate a major vector of email rejection. You’re not just preventing errors—you’re reinforcing trust with inbound mail systems that rely on consistent, valid authentication.

How MailTester helps detect and fix expired DKIM issues

When a DKIM signature expires, messages fail validation and risk rejection, especially with strict mail providers. MailTester’s real-time API and bulk verification tools detect expired or malformed DKIM signatures by analyzing email headers and DNS records, flagging problematic addresses before they cause bounces or damage sender reputation. This lets you clean your list and ensure deliverability.

Real-time verification catches DKIM errors early

Every time you send an email through the MailTester verification API, it checks the full email header—including DKIM signature validity, expiry status, and alignment with SPF and DMARC. If the x= tag shows an expired signature, it’s flagged immediately. This prevents sending to addresses where authentication has failed, reducing your bounce rate. Use the real-time API to validate addresses at scale without manual effort.

Bulk checks and inbox testing identify systemic risks

For large mailing lists, MailTester’s bulk verification scans thousands of emails, identifying those with expired DKIM signatures, malformed syntax, or missing keys. It flags these with a clear verdict, so you can remove or retry them. For deeper insight, inbox-placement testing sends real messages through actual inboxes—reporting DKIM status, spam scores, and delivery results. If a message fails DKIM, it’s highlighted in the report. This simulates real-world conditions, not just static checks. Check it all with the inbox tester.

After analysis, the in-app AI assistant reviews header data and DNS records to suggest fixes—like renewing a key, updating DNS TXT records, or checking alignment with the sending domain. It doesn’t guess; it cross-references known standards like RFC 6376 for DKIM and RFC 7672 for MTA-STS. These are proven mechanisms for validating email authenticity.

Understanding DKIM is key. As outlined by the IETF, DKIM signatures must be time-bound and correctly signed to prevent spoofing. When they expire, even legitimate messages risk being filtered. MailTester helps you maintain alignment with those standards. No guesswork. No false positives. Just actionable data and a clear path to fix issues.

Common signs your domain is affected by expired DKIM

If your emails are failing DKIM checks, you'll see failed verifications, bounces, and inbox placement drops. The x= tag in DKIM signatures indicates expiration, and when it's past its expiry, mail servers reject the message. This isn't just a technicality — it's a signal that your domain’s trustworthiness is compromised.

Immediate red flags in your email flow

  • Receiving non-delivery reports (NDRs) that say “DKIM signature verification failed” — this is the most direct sign your signature has expired.
  • Valid email addresses that were previously deliverable now return as invalid or risky in bulk verification — especially when you use tools like MailTester’s bulk verification.
  • Sudden drops in inbox placement during campaigns, even with clean lists and compliant content — often due to failed authentication, not content.
  • Spam scores rising on tools like ReturnPath or Spamhaus, even if no other changes were made to your sending practices.

How to diagnose the root cause

When DKIM fails, it’s not always obvious from the outbound logs. The x= tag in the signature is a cryptographic TTL; once it expires, the signature is considered void. This is standard behavior — referenced in RFC 6376, the core DKIM specification.

Let’s say you’re using a third-party email sender with an auto-renewal setup. If their key rotation fails or your DNS record isn’t updated, the signature is effectively broken — even if the key is technically “correct.”

Use MailTester’s inbox-placement test to simulate delivery across major inboxes. If deliverability drops across Gmail, Outlook, Apple Mail, and others despite clean content and list hygiene, DKIM failure is a likely culprit.

Even if only one address fails in an otherwise clean list, trace the DNS record. The x= expiration parameter is often overlooked in monitoring — but a single expired signature can trigger mass filtering.

DKIM validation isn’t just a checkbox. It’s a continuous trust signal. An expired signature breaks that chain.

Once you confirm DKIM is failing, check your DNS records, verify key rotation is active, and re-sign your messages. You can test the fix with MailTester’s real-time API, which checks both syntax and real-time validity — including expired signature detection.

Best practices for maintaining valid DKIM signatures

Prevent DKIM signature expired x= tag errors by using dynamic signing with per-email timestamps, rotating selectors regularly, keeping DNS records in sync, and monitoring delivery health. These steps ensure your emails stay authenticated and trusted, even as domains, keys, or infrastructure evolve over time.

Key actions to avoid expired DKIM signatures

  • Enable dynamic DKIM signing: ensure every email includes a fresh x= timestamp. This prevents expiration errors by aligning the signature’s validity with the message’s actual send time. Most modern ESPs handle this automatically—verify your setup is not using static keys.
  • Rotate DKIM selectors every 30–90 days. Using the same selector (e.g., default) long-term increases exposure if the key is compromised. Rotating reduces predictability and limits damage from a breach, following industry guidance like RFC 6376.
  • Keep DNS records synchronized between your email provider and domain registrar. A mismatch—e.g., outdated or missing DKIM TXT records—breaks authentication. Use tools like MXToolbox to validate DNS entries across both platforms.
  • Monitor your sender reputation and DKIM health using a dedicated deliverability dashboard. Real-time visibility helps catch errors like expired signatures, missing SPF, or DNS changes before they impact deliverability.
  • Test email delivery and inbox placement regularly. Use MailTester’s inbox placement tool to simulate how your messages appear across major inboxes, including those with strict filtering policies that may reject expired or malformed signatures.

Automate verification to reduce manual oversight

Use your email verification API to validate the integrity of your recipient list and catch invalid or problematic addresses early. A clean, authenticated send list reduces the risk of hitting DMARC policies that may flag unverified or misconfigured messages. Integrate MailTester’s real-time verification API with your CRM or email service to automate list hygiene and prevent send failures.

For large campaigns, use bulk list verification to assess the health of your email database. This helps identify records with outdated or expired DKIM-related configurations—especially critical in campaigns involving old or shared domains. Regular checks ensure your DKIM setup stays robust in the face of evolving email security requirements.

Why static expiration in DKIM is a design flaw

Static x= values in DKIM signatures create predictable failure windows—even when content is valid—because they lock expiration to a fixed time, rejecting emails after that point regardless of sender intent. This breaks automation, forces manual fixes, and causes avoidable bounces in high-volume campaigns. The DKIM standard doesn’t require x=, but most providers now expect it, turning a flexibility into a liability.

Expired signatures block valid messages

When a DKIM signature uses a static x= tag, say, with a 7-day expiration, emails sent after that window are rejected—even if the key is still valid and the message is not spam. This window is not dynamic; it doesn’t adapt to delivery delays, inbox caching, or retry schedules. The result? Legitimate emails get dropped by email providers like Gmail or Microsoft, simply because of an outdated timestamp.

Let’s say you send a time-sensitive alert at 10 a.m., but delivery retries kick in later. If the x= tag expires at noon, the retry fails. No content change. No sender fault. Just bad timing baked into the signature. This isn’t rare—it’s common in systems that haven’t updated their DKIM management practices.

Manual fixes don’t scale

Fixing static expiration requires regenerating the signature and re-signing every message after the cutoff. That’s not trivial. For a team sending 100,000 emails a day, this becomes a manual fire drill every week. Mistakes happen. Keys are misaligned. Expiry dates are entered wrong. The risk of failure increases with volume.

Many providers, including Microsoft and Google, now treat a missing or expired x= tag as a sign of poor operational hygiene—even if the core signature is correct. Standards like RFC 6376 do allow x= to be omitted, but today’s filters often flag its absence as suspicious. This expectation, despite the RFC’s flexibility, turns a design allowance into a default requirement.

Automated systems—like those in Mailchimp, HubSpot, or SendGrid—rely on consistent signing behavior. Static timestamps disrupt this, especially in long-running campaigns or triggered flows. The solution isn’t more monitoring; it’s avoiding static expiry entirely. Dynamic or extended expiry windows, or omitting x= when feasible, are better long-term practices.

Use tools like MailTester’s bulk email verification to check which senders in your list have outdated or malformed DKIM records. You can test delivery behavior in real inboxes with our inbox-placement tester, ensuring your mail doesn’t fail due to outdated signatures.

DKIM vs SPF vs DMARC: roles in email verification

You need SPF, DKIM, and DMARC together to validate email authenticity and maintain sender reputation. SPF checks if the sending IP is authorized. DKIM verifies the email content hasn’t changed and comes from the domain. DMARC uses both to enforce policies and report failures. If any one fails — especially an expired DKIM signature with an x= tag error — DMARC alignment breaks, causing delivery failure.

How each protocol works in practice

Let’s break down what each does and why it matters during verification:

Protocol What It Validates How It Works Impact on Verification
SPF Sender IP authorization Checks if the sending IP is listed in the domain’s DNS record as authorized. Failure means the email appears to come from an untrusted source, increasing bounce risk.
DKIM Content integrity and sender origin Digitally signs the email using a private key; validated with a public key in DNS. An expired DKIM signature (with x= tag error) breaks alignment, even if SPF passes — DMARC fails.
DMARC Policy enforcement and reporting Uses SPF and DKIM results to decide what to do with non-compliant emails: quarantine or reject. Single DKIM failure can result in DMARC policy enforcement, even if SPF is valid.

These protocols don’t operate in isolation. A single expired DKIM signature can invalidate DMARC alignment, leading to hard bounces or inbox filtering — even if the email content and IP are legitimate. This is why systems like MailTester check all three during bulk verification.

Why real-time checks matter

SPF, DKIM, and DMARC records change. Keys expire. IPs shift. You can’t rely on one snapshot. That’s why you need to verify email addresses in real time, not just test a static domain setting.

MailTester’s real-time verification API checks SPF, DKIM, and DMARC alignment in a single call. It tells you not just if an email is valid — but whether it’s authorized, signed, and aligned with current policies. Run a full inbox-placement test to see how your messages land in real inboxes, or verify a list of 100,000+ addresses with our bulk tool.

For more context on how email authentication works, see the IETF RFC 7052 on email authentication best practices. Also, the Spamhaus Project maintains lists that track domains violating these standards.

Verify emails with our API, or test a full list — and see which addresses fail due to DKIM, SPF, or DMARC issues before you send.

Don’t wait for delivery failure—test before you send

Invalid or expired DKIM signatures, including x= tag errors, can break your email delivery before it leaves your server. The only way to catch these issues early is to test your setup before sending to real users.

Prevent issues with proactive validation

Use MailTester’s bulk verification to scan your entire list for invalid or catch-all addresses. Catching misconfigured DKIM signatures at scale stops bounces and protects your sender reputation before they impact deliverability.

Test campaigns before they go live

Run inbox-placement tests to simulate how your message lands in real inboxes. These tests expose DKIM, SPF, and DMARC misconfigurations—common sources of the "DKIM signature expired x= tag error"—before you send to your audience.

Integrate verification into your workflow

  • Connect MailTester’s real-time API with your CRM, Mailchimp, HubSpot, Klaviyo, or SendGrid.
  • Automate list hygiene by verifying new signups and flagged addresses on the fly.
  • Keep your sender reputation strong by eliminating risky or unreachable emails from your campaigns.

Sources

Keep reading

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

Frequently asked questions

What does DKIM signature expired x= tag mean?

It means the DKIM signature's expiration timestamp has passed. The receiving server rejects the signature as invalid, even if the email content is unchanged.

Can an expired DKIM signature prevent email delivery?

Yes. Most major email providers reject messages with expired DKIM signatures, especially if SPF and DMARC also fail.

How often should DKIM signatures be renewed?

They should be renewed with every send. Static or long-lived timestamps cause expiration errors and reduce deliverability.

Does MailTester check DKIM signatures during verification?

Yes. MailTester’s real-time API and bulk verification scan email headers for DKIM validation, including expiration status.

Why does my email still pass SPF but fail DKIM with x= expired?

SPF validates the IP, but DKIM validates the content and origin. An expired DKIM signature breaks authenticity, even if the sending IP is valid.

Can I fix an expired DKIM signature without touching DNS?

Only if your email service auto-generates signatures. You can't fix it manually without updating the signature in the email header.

Is the x= tag required in DKIM?

No. It is optional. But providers like Gmail and Outlook now expect it and use it to enforce validation.

How do I know if my DKIM record is misconfigured?

Use tools like MailTester or MxToolbox to test your DNS records and verify that signatures are active and not expired.

What’s the impact of ignoring expired DKIM errors?

It leads to higher bounce rates, lower sender reputation, and increased spam filtering—even if your content is legitimate.

Can a catch-all email address mask expired DKIM issues?

No. Catch-all addresses may accept the email, but delivery to the intended mailbox still fails if DKIM is expired and rejected.