Why does your email verification API return 451 4.7.5 for valid addresses?

You send a verification request to your email verification API. The result comes back: 451 4.7.5. The address is supposedly invalid. But you know it isn’t. You’ve sent to it before. It’s in your records. The bounce code seems to contradict reality. What’s really going on?

That 451 4.7.5 error isn’t about the recipient address—it’s about your own sending setup. It means the receiving server couldn’t validate your message due to a temporary policy or authentication failure. Specifically, the sender’s domain failed DKIM verification because the digital signature key had expired. The address is valid. Your verification is right. But your infrastructure isn’t.

This isn't a flaw in the verification system. It’s a red flag about your own email authentication. You're being blocked not because you're sending to bad addresses, but because you're authenticated poorly.

Key takeaways

  • A 451 4.7.5 error during SMTP verification indicates a temporary rejection due to sender-side authentication failure, not recipient invalidity.
  • Expired DKIM keys on your domain can cause valid addresses to appear as unreachable, even when the email itself is correct.
  • Email verification APIs return 451 4.7.5 for valid addresses when your sending domain lacks up-to-date DKIM configuration, revealing gaps in your outbound email setup.

What does SMTP error 451 4.7.5 mean in email verification?

SMTP error 451 4.7.5 means the receiving server temporarily rejected your email during delivery because of a policy, security, or authentication issue—specifically, it failed to validate the DKIM signature. This isn’t a permanent problem with the email address, but a signal that the server is applying temporary restrictions, often due to expired or misconfigured email signing keys. In email verification, this usually means the test message was sent successfully, but the recipient's server rejected it because DKIM validation failed.

Why DKIM matters in email delivery and verification

Digital signatures via DKIM authenticate that an email wasn’t altered in transit. When a domain’s DKIM key expires or is misconfigured, receiving servers—especially strict ones like Gmail or Microsoft 365—may reject messages with a 451 4.7.5 response, even if the address exists. This is a transient issue, not a sign the email is invalid.

Let’s clarify: the error occurs during the mail transaction phase, after the connection is established and the protocol reaches the actual delivery step. That’s why verification tools like MailTester can detect this—they’re simulating a real delivery attempt and recording the server's response in real time. If the API returns 451 4.7.5, it’s not because the email is fake, but because the domain’s security mechanism failed at the moment of check.

How to act on a 451 4.7.5 error in verification

You shouldn’t treat 451 4.7.5 as a hard bounce. It’s temporary. Many large providers will retry delivery, and the message might reach the inbox later. But in verification contexts, this signal is still valuable—it exposes underlying deliverability risks like expired cryptographic keys, which hurt sender reputation over time.

For example, a domain with an outdated DKIM key may pass email hygiene checks, but still trigger rejection across multiple inboxes. Tools that return this code during verification help you surface those hidden issues before sending to hundreds of users.

Some email providers, including Google and Microsoft, document these error codes in their official RFCs and post-mortem reports. The DKIM specification defines how signatures are validated, and Google’s Mail Essentials page explains how servers handle rejected messages due to authentication failures.

If you’re testing sender health or validating lists at scale, real-time verification via an API like MailTester’s email verification API can flag these transient issues early—before they impact your deliverability. You’re not verifying a single address, but identifying systems that may fail on actual delivery, even if they’re not technically invalid. That’s more than just a bounce—it’s a warning sign.

How expired DKIM keys cause 451 4.7.5 in real-time email verification

When your email verification API returns a 451 4.7.5 error, it often means the recipient server rejected your message because the sender’s DKIM key has expired. DKIM signatures are time-limited cryptographic proofs; once expired, even legitimate emails fail validation. During a real-time verification check, the receiving server tests the full email transaction—including the DKIM signature—and blocks messages with expired keys. This is a common reason for failed deliveries, even when the address itself is valid.

Digital signatures have expiration dates

DKIM keys aren’t permanent. They’re certificates tied to a specific time frame, typically set by the domain owner. After that period, the key is no longer trusted—even if the domain, sender, and content are valid. This is a deliberate security measure. Receiving servers, like Gmail or Microsoft’s mail systems, check that signature against active keys on file.

For example, RFC 6376, the standard for DKIM, explicitly allows for key expiration as part of its design. The receiving server validates the signature’s timeliness as part of its filtering process. If the key is expired, the server rejects the message with a 451 4.7.5 code, which means "delivery temporarily deferred due to a policy violation"—in this case, expired authentication.

How verification APIs expose this flaw

When you use a real-time email verification API, it doesn’t just check if an address exists. It simulates a real email send by initiating full SMTP transactions: HELO, MAIL FROM, RCPT TO, and DATA. During the DATA phase, the server receives the full email, including the DKIM-Signature header. At that moment, it checks whether the key used to sign the message is still valid.

If the key has expired, the server will reject the message with a 451 4.7.5 error. This happens even if the recipient address is legitimate and the domain is active. The error isn’t about the address—it’s about the sender’s identity being unverified at the cryptographic level.

MailTester’s real-time API performs this exact process. It sends a test message to the receiving server and observes the response. If the server returns 451 4.7.5 due to an expired DKIM key, the API flags the sender as potentially compromised or misconfigured, even if the mailbox is active. This helps you detect issues before sending your main campaign.

Understanding this helps separate real deliverability risks from simple bounce issues. You can’t fix an expired DKIM key with a better email list—but you can catch it early with a high-accuracy verification tool like MailTester’s real-time API. Knowing the exact cause of a 451 4.7.5 error lets you diagnose sender-side problems before they cost you engagement.

How to diagnose if expired DKIM is causing your 451 4.7.5 errors

If your email verification API returns 451 4.7.5 and only your domain’s messages are failing, the issue likely isn't with the recipient’s server — it's with your own DKIM key. Test the same address from different senders; if only your domain triggers the error, your DKIM signature is probably expired. Check your DNS TXT records and your sending provider’s key rotation policy — many rotate keys every 90 days. If your key expired or isn’t properly published, the server rejects your email as untrusted.

Check DKIM key validity across your infrastructure

  • Test the same email address from a different sender. If only your domain returns 451 4.7.5, the problem is sender-side, not recipient-side. This isolates the issue to your domain's signing setup.
  • Use tools like MxToolbox or Spamhaus to look up your DKIM TXT record. Confirm the selector matches your sending setup and check for an expiration timestamp.
  • Review your domain’s TXT records directly in your DNS provider’s console. Locate the DKIM record and verify the key is still valid, not truncated, and matches the expected selector (e.g., default._domainkey.yourdomain.com).
  • Check your sending provider’s documentation for DKIM key rotation practices. Many platforms — including SendGrid, Amazon SES, and Outlook.com — automatically rotate keys every 90 days. If you haven’t updated the key in the DNS, it’s expired.
  • Validate the full DKIM signature path: the key must exist in DNS, be correctly formatted, and align with the From header. Mismatched or expired keys cause rejection at the receiving end.
  • If you’re using a custom domain with a third-party mailer, ensure your API or SMTP client is publishing the new key within the expected window. Delayed updates during rotation are a common cause of 451 4.7.5.

Automate checks to prevent future issues

Once you confirm the key is expired, replace it and wait for DNS propagation. But you can avoid repeat failures by verifying your DKIM setup regularly. Use MailTester’s email checker to test individual addresses and catch errors before they hit the inbox. For bulk validation, use the bulk verification tool to clean your list and spot sendability issues early.

DKIM validation failures like 451 4.7.5 indicate that the receiving server couldn’t verify the sender’s identity. The most common cause is an expired or misconfigured signature.

Fixing 451 4.7.5: Steps to resolve expired DKIM in email verification workflows

When your email verification API returns 451 4.7.5 due to an expired DKIM key, the root cause is a stale cryptographic signature in your domain’s DNS records. This error indicates that the receiving mail server rejected your message because the DKIM key used to sign it is no longer valid. You must regenerate and republish the key in your DNS provider’s dashboard, wait for propagation, then retest using a verified address from your domain.

Diagnosing the Expired Key

  1. Log in to your email service provider (ESP) or DNS dashboard. Access your domain’s DNS management through your provider (e.g., Cloudflare, AWS Route 53, GoDaddy) or your ESP’s admin panel (like SendGrid, Mailgun, or Amazon SES).
  2. Check your DKIM configuration and locate the active key record. Look for a TXT record with a selector (like default._domainkey or mail._domainkey) under your domain. This is the key used to sign outbound emails.
  3. Verify if the key has expired using a DKIM validator tool or DNS lookup. Use a tool like MXToolbox’s DKIM Checker or RFC 6376 to check the key’s validity and expiration date. If it’s past its validity window, it’s expired.

Regenerating and Publishing the Key

  1. Generate a new DKIM key pair in your ESP or DNS manager. In your ESP, navigate to the DKIM management section and create a fresh key. Most providers let you do this in seconds.
  2. Copy the public key string exactly as provided and publish it in your DNS. Paste it into a new TXT record using the same selector as before. Do not change the selector—it must match the one the verifier expects.
  3. Wait for DNS propagation. DNS changes typically take 5 to 30 minutes to reflect globally. Use DNS Checker to confirm the new record is live across the internet.
  4. Re-test the verification API using an address from your domain. Run a test with a real address from your sending domain (e.g., [email protected]) through your API. If properly fixed, the 451 4.7.5 error should no longer appear.

If the error persists, double-check the selector, record format, and propagation status. Some services require you to wait longer or manually re-activate the key in the dashboard.

Use MailTester’s real-time email verification API to validate your email list before sending and detect delivery issues early. It checks DKIM status, spam traps, and inbox placement—all in one check.

Why real-time verification APIs like MailTester still return 451 4.7.5 for expired keys

MailTester returns a 451 4.7.5 error during verification because it simulates a real email delivery attempt—testing not just whether an address exists, but whether it will actually be accepted by the recipient’s mail server. If the sender’s DKIM signature has expired, the receiving server rejects the message with 451 4.7.5, and MailTester surfaces that same rejection. This isn’t a false positive; it’s intentional. By catching expired DKIM keys early, MailTester flags a real deliverability risk before your email campaign launches.

How verification becomes a delivery preview

When you send a message via SMTP, the receiving server checks more than just the recipient’s address. It verifies SPF, DKIM, and DMARC—authentication mechanisms that confirm you’re authorized to send from that domain. An expired DKIM key breaks that chain. Even if the address is valid, a failed signature means the email will be blocked.

MailTester doesn’t skip these checks. It performs a full, real-time SMTP transaction with the remote mail server. This includes verifying the sender’s domain alignment and validating the DKIM signature during the connection handshake. The 451 4.7.5 error you see in the response reflects the actual outcome a real delivery would face.

Why this matters for sender reputation and inbox placement

Receiving a 451 4.7.5 during verification isn’t a flaw—it’s a feature. It reveals a risk that could otherwise go unnoticed until your campaign fails. A single failed authentication attempt can hurt your sender reputation, especially with providers like Gmail and Outlook that monitor alignment and signature validity closely.

According to industry standards, valid authentication is a foundational requirement for deliverability. The RFC 6376 specification details how DKIM signatures must be cryptographically valid and not expired at the time of delivery. If an email fails at any of these checks, the server must reject it—and that’s exactly what MailTester simulates.

Let’s say you’re sending to a large list. If even 1% of those emails are rejected due to expired DKIM, your sender reputation suffers. MailTester helps you catch those flaws before they cause damage. The 451 4.7.5 error is a signal, not a failure—proof your system is working, just as it should.

If you’re verifying emails at scale, this level of realism is critical. Use MailTester’s real-time verification API to validate not just addresses, but the full delivery pipeline—down to the most precise server response codes.

How to prevent 451 4.7.5 errors in bulk email verification

451 4.7.5 errors due to expired DKIM keys happen when your email server's cryptographic signature fails validation because the key has expired or wasn’t published in time. To stop this, audit your DKIM configuration regularly, monitor DNS records for changes, and verify your sending infrastructure before mailing. Use tools like MailTester to catch issues early in your send prep pipeline. This reduces bounce rates and protects sender reputation.

Proactively manage DKIM key lifecycles

  • Set calendar reminders for DKIM key expiration dates—most keys are valid for 90 to 365 days.
  • Use automated monitoring tools to check DNS records for correct DKIM TXT entries and detect gaps before they trigger bounces.
  • Test your DKIM setup using a tool like MXToolbox or RFC 6376 (the standard for DKIM) to ensure signatures are generated and published properly.

Integrate early validation into your workflow

  • Before sending bulk emails, run every address through a real-time email verification API to catch invalid or non-receiving addresses—and identify DKIM-related issues in your sending domain.
  • Use MailTester's API during your send prep pipeline to flag domains with expired or misconfigured DKIM before they cause 451 4.7.5 errors in production.
  • Always publish a new DKIM key well before the old one expires—never let the old key stop working before the new one is active, to avoid overlap gaps.
  • Verify that both the public key and selector are correctly published in DNS, as missing or incorrect entries lead directly to authentication failures.

What does MailTester do differently when a 451 4.7.5 occurs?

When your email verification API receives a 451 4.7.5 error, MailTester doesn’t treat it as a final verdict on the email address. Instead, it flags the result as ‘risky’ or ‘server error’ based on context, including the full SMTP rejection reason. This distinction helps you tell whether the issue is with the recipient’s mail server configuration—like an expired DKIM key—or with the email address itself.

It keeps the signal clean by preserving the full rejection reason

Every time a 451 4.7.5 response occurs, MailTester captures the full SMTP error code and includes it in the response payload. This means you see not just the code, but the actual reason—such as 'expired DKIM key' or 'temporary policy violation'—right alongside the address. You’re not left guessing whether the failure is temporary or permanent.

Let’s say you’re doing a bulk verification and a large number of addresses trigger 451 4.7.5. Without context, you might assume they’re all invalid. But MailTester’s granular classification separates those that are likely valid (just facing a temporary server issue) from those that aren’t. This is crucial during list hygiene audits, where you need to differentiate between a bad email and a bad infrastructure setup.

Filter out noise, focus on what matters

With this data, you can filter out the 451 4.7.5 results in your bulk reports and focus remediation on addresses that are truly invalid or risky. The API does not default to marking every 451 4.7.5 as “invalid”—it respects what the server says. As the IETF notes in RFC 5321, 4xx SMTP codes indicate temporary failures, not permanent ones—this is not a new discovery, but it’s often ignored in automated systems.

By treating 451 4.7.5 as a signal to investigate, not discard, you avoid prematurely discarding valid addresses. It also means you can identify broader infrastructure issues—like a misconfigured DKIM policy—before they affect delivery at scale. You’re no longer triaging lists with guesswork; you’re acting on clear, context-rich data.

If you’re checking email validity before sending, you can test with our real-time email checker to see if a specific address triggers 451 4.7.5 during delivery. For larger systems, the API returns full diagnostic details, helping you automate intelligent filtering across your campaigns.

DKIM, SPF, and DMARC: How they interact during verification

When an email verification API returns a 451 4.7.5 error, it typically means the recipient server rejected your message due to a DKIM signature failure—often because the public key has expired. But SPF or DMARC misconfigurations can also trigger this outcome. These three protocols work together: SPF checks the sending IP, DKIM verifies message integrity, and DMARC enforces policies based on their results. A failure in any one can block delivery, even if the email address itself is valid.

Why 451 4.7.5 Often Points to DKIM

Most 451 4.7.5 errors during verification stem from expired or invalid DKIM signatures. This happens when the domain’s DKIM key has been rotated or removed without updating the DNS record. Without a valid signature, the receiving server flags the message as tampered or untrusted, even if the address and sender IP are correct. It’s a common issue when organizations update email infrastructure but forget to renew their cryptographic keys.

How SPF, DKIM, and DMARC Work Together

SPF validates that the sending server’s IP is authorized by the domain's DNS records. DKIM ensures the message body and headers haven’t changed during transit by signing the content with a private key. DMARC acts as the policy engine, deciding what to do when SPF or DKIM checks fail—like quarantining or rejecting the message. Email verification tools simulate actual delivery and test all three. If any one fails, the API may return a 451 or a similar SMTP error.

That’s why testing a single address isn’t enough. A 451 4.7.5 error doesn’t always mean the address is bad—just that the technical setup around it is broken. For example, a legitimate user might get blocked if their organization’s DKIM key expired months ago, even if they’re a real person.

Tools like MailTester test all three protocols in parallel during verification, giving you a complete view of deliverability health. You can check whether SPF is configured, if DKIM signatures are valid, or if DMARC policy is strict enough to cause rejection. This full-stack check helps you catch problems before they hit your inbox placement.

You’re not just validating addresses—you’re validating the entire email infrastructure. This is why a tool that analyzes SPF, DKIM, and DMARC together is more useful than one that only checks inbox existence. If you’re running campaigns or syncing with platforms like HubSpot or SendGrid, ensuring these records are correct matters just as much as having a real address.

Learn how MailTester verifies email lists with full protocol checks: verify your entire email list with SPF, DKIM, and DMARC validation.

Drafts and templates aren’t the only things that break during delivery. The authentication stack is a known failure point—and a growing one. See the official RFC 6376 for the full DKIM specification, and RFC 7052 for DMARC implementation guidance. These are the standards your email system must follow, not just wishful thinking.

How to avoid false positives in email verification due to outdated keys

When your email verification API returns a 451 4.7.5 error due to an expired DKIM key, it’s not necessarily because the email is invalid. The error reflects a configuration issue on your end, not the recipient’s. Never treat verification results as absolute—always cross-check against your current DNS records. Use inbox-placement testing to see if messages from your domain actually reach inboxes, and combine this with sender reputation checks to isolate whether failures stem from infrastructure or your list quality. Set up alerts for DNS changes and key expiration to catch issues before they disrupt deliverability.

How to validate results safely

  • Don’t trust verification outcomes blindly—many tools report expired DKIM keys as invalid addresses, even when the mailbox is live. Your domain’s current DNS setup is the final authority.
  • Use inbox placement testing to simulate real deliveries from your domain and validate whether your authentication and infrastructure are working end-to-end.
  • Check both the recipient's address and your own sending infrastructure. A 451 4.7.5 error often points to a misconfigured or expired DKIM signature, not a bad email.
  • Verify your DKIM records with tools like MxToolbox or RFC 6376 to ensure they’re properly registered and not expired.

Proactive monitoring is essential

  • Set up alerts for DNS record changes—especially for SPF, DKIM, and DMARC. A small change during a migration can break authentication.
  • Track DKIM key expiration dates manually or via automated tools. Keys typically expire every 1–2 years, but missing a renewal means sudden bounces.
  • Use APIs that return detailed verdicts (like MailTester’s verification API) to get insight into why an address is flagged—whether it’s a key issue, a catch-all, or a role account.
  • Combine list verification with sender reputation checks to distinguish between a bad list and a bad system. If your domain reputation is healthy but many emails fail, it’s likely not your data.
  • Monitor blocklist status regularly using tools like Spamhaus or Google Postmaster Tools, as poor sender reputation can mimic delivery failures from expired keys.
False positives in verification aren’t just inconvenient—they erode trust in your list management. The real fix is transparency: don’t assume the API told you the whole story.

Conclusion: 451 4.7.5 is a signal — not a blocker

A 451 4.7.5 error during verification isn’t a red flag for the recipient email. It’s a signal about your own sending infrastructure.

Specifically, it indicates your DKIM key has expired. This does not mean the email is invalid — it means your domain’s authentication setup is outdated and may cause deliverability issues.

Correcting the DKIM key and re-testing with a tool like MailTester ensures your messages are trusted by receiving servers. The result: inbox placement, not rejection or spam filtering.

MailTester’s 98.9% accuracy detects these technical issues early. You’re not just validating addresses — you’re auditing your sending setup before you send a single message.

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Frequently asked questions

What does 451 4.7.5 mean in email verification?

SMTP 451 4.7.5 means the receiving server temporarily rejected the email due to a policy or authentication failure. It often reflects expired DKIM keys, not invalid addresses.

Why is my email verification API returning 451 4.7.5 for valid emails?

This happens when the sender's DKIM key has expired. MailTester performs full SMTP tests, including signature validation, so the error surfaces infrastructure issues.

Can DKIM expiration affect all emails from my domain?

Yes. Expired DKIM keys cause receiving servers to reject messages, leading to bounces, low inbox placement, and sender reputation damage.

How often should DKIM keys be rotated?

Most providers recommend rotation every 90 days. Check your email service provider's documentation for specific policies.

How can I test if my DKIM key has expired?

Use a DKIM validator tool or check your domain’s DNS TXT records. Look for the key’s expiration date in the DNS payload or validate via email header analysis.

Does MailTester confirm whether my DKIM key is valid?

MailTester doesn’t validate DNS records directly. It tests DKIM by simulating email delivery — if the key is expired, the server returns 451 4.7.5.

What happens if I ignore 451 4.7.5 errors during verification?

You’ll send to domains that reject your messages due to expired keys. This harms sender reputation, increases bounce rates, and hurts deliverability.

Is 451 4.7.5 a permanent error?

No. It’s a temporary rejection. The error will resolve once the DKIM key is updated and the DNS record propagates.

Can email verification APIs like MailTester help prevent deliverability issues?

Yes. They simulate real delivery and detect technical issues like expired DKIM keys, catch-all domains, and misconfigured SPF before sending.

How accurate is MailTester’s email verification?

MailTester achieves 98.9% accuracy across all verdicts including valid, invalid, catch-all, and risky addresses.

Do MailTester credits expire?

No. All purchased verification credits never expire, giving you flexibility to use them when needed.

Can I integrate MailTester with SendGrid or Mailchimp?

Yes. MailTester integrates directly with Mailchimp, HubSpot, Klaviyo, and SendGrid to automate list hygiene and verification.