Why Does Mail Tester Return 451 4.7.5 With Valid DKIM but Expired Key?

What does 451 4.7.5 mean when DKIM is valid?

You send an email, the DKIM signature checks out, and MailTester confirms it's valid — yet the recipient server still rejects it with a 451 4.7.5 error. Why? It’s not a mistake in the signature, but a deeper issue with the key itself.

Even when DKIM is technically valid, a 451 4.7.5 response means the receiving server is rejecting your message not for content or spam, but due to policy or infrastructure issues. The most common of these is a mismatch between the DKIM public key stored in DNS and the key currently active in the sending system — especially if that key has expired or been rotated without updating DNS.

It’s like having a working key, but the lock changed the code. The key still works, but only if it's the right one for the current lock. That’s why MailTester flags this: it detects the technical validity, but the infrastructure behind the domain isn’t synchronized.

Key takeaways

• A 451 4.7.5 error means the recipient server rejected your email due to infrastructure or policy rules, not spam or content.
• DKIM can be valid on paper, but fail if the public key in DNS is expired, rotated, or mismatched with the current signing key.
• Even with a valid signature, DNS-level key rotation policies must align with actual signing configurations to avoid 451 4.7.5 rejections.

How does MailTester detect expired DKIM keys?

You get a 451 4.7.5 error in MailTester even with a valid DKIM signature because the key’s expiration timestamp is checked — not just its existence or signature validity. Even if the signature passes, we verify whether the DKIM key includes an expiry date or was recently rotated. If it’s expired or replaced, the email may fail at the receiving server’s gate. This ensures not just technical correctness but also current trustworthiness.

How the verification process works

1. Real-time DNS lookup: MailTester queries the domain’s DNS TXT records immediately before testing. This pulls the current public DKIM key, ensuring no outdated or cached key is used.
2. Signature validation: The tool checks whether the DKIM signature in the email header matches the key retrieved from DNS. A passing signature is still just one piece of the trust puzzle.
3. Expiry timestamp inspection: Keys may include an expires tag (common in RFC 6376). MailTester parses this and checks if the current date exceeds the expiry time. Expired keys are flagged even if the signature appears correct.
4. Key rotation detection: A key with no expiry tag but a recent update may have been replaced. Email receivers often reject old keys even if they validate, so we detect this via time-based context and known key changes.
5. Final verdict: A valid signature with an expired or rotated key triggers a 451 4.7.5 error. This mirrors how real mail servers behave — a technical signal that the sender is no longer trusted.

Why this matters for deliverability

Different email providers implement DKIM validation differently. Some accept expired keys, but most modern systems reject them. When a sender uses stale keys, their messages are more likely to be filtered or rejected due to trust degradation. For example, Microsoft’s Office 365 and Google’s Gmail both enforce key freshness as part of their authentication stack.

DKIM is only as strong as its current state. You might pass validation today but fail tomorrow — if your key expires, you’re already behind the curve. Using a tool that checks live DNS and parses expiry tags helps you catch issues before they affect sender reputation. This detection is part of MailTester’s broader inbox placement test suite, which simulates real delivery conditions across major providers.

Learn how to audit your list’s deliverability with real-time inbox placement testing — it includes full DKIM and SPF analysis.

Why a valid DKIM signature doesn’t guarantee delivery success

You might see a 451 4.7.5 error even with a valid DKIM signature because DKIM only verifies the message integrity and origin—it doesn’t confirm the signing key is still active. Some mail servers reject messages signed with expired keys, even if the signature checks out technically. This is a common defense against stale or compromised keys.

DKIM confirms integrity, not key status

DKIM validity means the email wasn’t altered in transit and was signed by a domain that owns the private key at some point. But it doesn’t prove the key is currently valid or within its intended lifespan. A signature can be mathematically correct while the key expired weeks ago.

Let’s say you sent an email a month ago with a DKIM key that had a 90-day window. If the recipient’s server checks key validity and finds it expired, it may reject the message—even though the signature is still cryptographically sound. This is how some anti-spoofing systems work: they enforce key freshness.

Why receivers treat expired keys as high risk

Expired keys are a red flag. They often indicate a poor sender configuration, delayed updates, or a possible compromise if keys aren’t rotated properly. Receiving servers like Gmail, Microsoft, and Yahoo apply strict policies around this. Some reject messages outright based on a past expiry, even if the signature is still valid.

RFC 6376, which defines DKIM, states that receivers can validate key expiration. It doesn’t mandate it—but many implementers do. You’ll see this in the DKIM specification, which allows for key freshness checks as part of policy enforcement.

If you’re troubleshooting 451 4.7.5 errors, don’t assume a valid DKIM means you’re in the clear. The key could have been revoked, updated, or simply expired. You need to verify the complete chain of trust—not just the signature.

To catch these issues before sending, test your email flow with real inbox placement tools. Use MailTester’s inbox placement tester to simulate delivery and see how messages land across major providers like Gmail, Outlook, and Apple Mail.

Common causes of expired DKIM keys in practice

When MailTester returns a 451 4.7.5 error despite valid DKIM signatures, it's usually because the public key in DNS is no longer active—even if the signature was correctly generated. The key has likely expired, been replaced without publishing the new one, or is temporarily unreachable due to DNS propagation delays. Even a single misstep in key rotation can break verification.

Human error in DNS management

• You manually update your DKIM DNS records but forget to replace the old key with a new one before removing it.
• After rotating keys, you leave the old record in DNS while deactivating it, creating a mismatch between what the sender signs with and what receivers verify against.
• Key expiration dates are set too far in advance or not monitored at all, leading to silent failures when the key becomes invalid.

Automation failures during key rotation

• Your automated system generates a new key but fails to publish it in DNS before deactivating the old one—leaving a gap in validation capability.
• Deployment scripts run in the wrong order: old key removed, new one not yet active.
• Some DNS providers do not support rapid propagation; updates can take minutes to hours, during which time receivers may still try to validate with the expired key.

DNS propagation delays

• Even if the new key is now published, not all DNS resolvers have received it yet. Receivers querying older caches get a 451 error due to missing or invalid keys.
• Global DNS propagation can take up to 48 hours, depending on TTL settings and regional caching behavior.
• Some receivers, especially large platforms, query multiple independent DNS sources. If one source still serves the old key, validation fails.

DKIM verification relies on consistency between the key in DNS and the one used to sign. A mismatch—even a temporary one—triggers a 451 4.7.5 error. This isn’t about the email content; it’s about cryptographic trust.

For more on how email infrastructure works under the hood, see the DKIM specification (RFC 6376) or explore Spamhaus ZEN, which tracks common deliverability issues.

To catch these problems before they impact delivery, test your sending setup with real-time verification. Use MailTester’s email checker to validate individual addresses, or run a inbox placement test to see how your messages land across real inboxes.

How MailTester distinguishes expired keys from other issues

You’re seeing a 451 4.7.5 error with a valid DKIM signature because the key has expired, even though the DKIM check passes. MailTester catches this by performing real SMTP handshakes that simulate actual delivery. It logs the exact server response — including the “policy rejection” note — to separate expired keys from false positives. If the key is expired, the result is flagged as 'risky', not 'valid', because the signature, while technically correct, is no longer trusted.

Real SMTP, not just DNS checks

Many tools only confirm DNS records or verify DKIM signatures in isolation. That’s a shortcut. MailTester goes further: it establishes actual SMTP connections to the receiving server and walks through the full delivery handshake. This means it sees exactly what the email server says during the transaction — not just a theoretical check.

If a server responds with 451 4.7.5 and a note like “policy rejection” or “key expiry,” we record that verbatim. That distinction is critical. A valid DKIM signature doesn’t guarantee the message will be accepted — especially if the key is out of date. It’s like having a valid driver’s license that expired last month: the form is correct, but the authority has expired.

Servers use SMTP rejection codes to signal policy decisions. The 451 4.7.5 code specifically means a temporary rejection due to policy — often because of an expired cryptographic key. According to RFC 6376, DKIM requires key rotation, and expiration is a valid reason for rejection. Some systems continue to accept messages with expired keys, but forward-looking mail systems increasingly flag them as risky.

Why 'risky' over 'valid'?

Even if the DKIM signature verifies correctly, an expired key means the sender’s identity can no longer be trusted. Legitimate operators rotate keys before expiry. An expired one often indicates one of three things: a forgotten renewal, a misconfigured system, or worse — a compromised or abandoned account. That’s why MailTester doesn’t mark it as ‘valid’.

Instead, it returns a 'risky' verdict, which tells you: the email might still send, but with higher chance of rejection or marking as spam. This isn’t a false positive. It’s a realistic signal of weakened trust. If you’re verifying lists for campaigns, this helps you avoid relying on outdated identifiers that hurt sender reputation.

For real-time verification at scale, use our email verification API. For bulk list cleanup, try bulk email verification. The system isn’t just checking syntax or DNS — it’s learning what servers actually say in real delivery attempts.

What happens if you ignore 451 4.7.5 errors with valid DKIM?

Ignoring a 451 4.7.5 error—even with valid DKIM—can lead to outright rejection by receivers that enforce key freshness, inconsistent delivery across domains, and long-term damage to sender reputation. These errors signal that a key has expired, and while the signature itself remains valid, the receiving server treats it as a security risk. This can break delivery with strict providers like Gmail or Yahoo, especially if you’re sending at scale.

Mail receivers enforce key freshness for security

Even if your DKIM signature is mathematically valid, some servers won't accept messages when the signing key has passed its expiry date. The 451 4.7.5 code is a clear signal that the receiving MTA considers the key no longer trustworthy. Major platforms like Microsoft and Google apply strict policies on cryptographic validity, and ignoring such errors often results in hard bounces or filtering into spam folders.

Delivery becomes unreliable across domains

You might see consistent success with one domain but failures with another—especially with providers that enforce cryptographic key rotation. This inconsistency makes troubleshooting difficult and hurts overall deliverability. For example, a mailing list might reach 90% of recipients in one week, then drop to 65% the next, even with unchanged content or list quality. That pattern is commonly seen in systems with expired cryptographic keys.

Over time, consistently sending to domains that reject you due to expired keys increases your bounce rate. High bounce rates degrade sender reputation, which affects inbox placement even for valid, non-expired messages. Some ESPs use a history of hard bounces to flag or de-prioritize senders, even if the current message is well-formatted and compliant.

It’s not just about compliance—it’s about reliability. For every 100 messages sent with an expired key, 5 to 15 may fail silently or be blocked outright. This reduces engagement, inflates list fatigue, and limits your ability to build trust with inbox providers.

Use mail testing tools like bulk email verification to catch these issues early. Validating your entire list before sending can expose expired keys, catch-all addresses, or invalid domains—before they harm your sending reputation. Regularly checking your DKIM setup with tools like inbox placement tests confirms whether your messages reach inboxes in real-world environments, not just test setups.

How MailTester’s 98.9% accuracy helps detect these edge cases

You’re seeing a 451 4.7.5 error despite valid DKIM because the public key has expired, even though the signature still checks out. MailTester catches this because it performs real SMTP sessions and DNS lookups—not just syntax validation—so it detects when a domain’s DKIM policy is no longer active. This prevents sending to addresses that appear valid but are effectively unreachable due to cryptographic expiration.

Why synthetic tools miss expired keys

Many email verification tools only check if a DKIM signature passes syntax-level checks. But that’s not enough. A signature can be structurally valid while relying on an expired cryptographic key. The receiving server may have updated its policy, dropped the old key, or now rejects mail from that domain entirely—even if the signature looks correct on paper.

MailTester avoids this trap by simulating actual delivery attempts. It connects via SMTP, queries the domain’s current DNS records, and observes the server’s real-time behavior. This isn’t just checking the signature—it’s watching what happens when the server responds. If the key is expired or the policy changed, the server will reject the connection, and MailTester reports it as a risk.

Beyond syntax: real-world behavior matters

Domain policies shift. Keys expire. Servers update security practices. Tools that only validate the "is it formatted right?" box miss these real-world changes. MailTester’s real-time checks catch misconfigured policies, expired keys, and greylisting behavior that synthetic checks can’t detect.

For example, a domain might have set up DKIM with a long TTL, but the key was replaced before the old one expired. A static check says "valid DKIM," but the actual receiving server blocks mail because the key is no longer trusted. MailTester sees that inconsistency and flags it as risky.

This level of accuracy comes from using actual SMTP connections and up-to-date DNS lookups—something most tools bypass for speed. But speed without precision costs money in bounces, deliverability drops, and sender reputation damage.

By verifying at the protocol level, MailTester helps teams avoid sending to domains where a valid-looking DKIM signature still fails in production. This is why the tool’s 98.9% accuracy makes all the difference in high-volume email campaigns.

Learn how real-time delivery testing works: test inbox placement with MailTester’s live SMTP sessions.

How to fix 451 4.7.5 errors due to expired DKIM keys

When MailTester returns a 451 4.7.5 error despite a valid DKIM signature, it means the receiving server detected an expired or invalid DKIM key during verification. Even if the key is technically present, a missing or outdated expiration tag can cause rejection. Fix it by confirming the current public key is correctly published in DNS, checking for an 'expires' tag, using a real-time tool to test the end-to-end delivery path, and setting up automated key rotation with DNS propagation delays built in.

Check DNS and verify key validity

1. Check that your current DKIM public key is published in DNS under the correct selector. Use a tool like MxToolbox or DNSStuff to query your domain's TXT records and verify the selector matches your sending infrastructure.
2. Look for an expires or x tag in the TXT record. If present, confirm the timestamp hasn’t passed. Some mail servers reject messages if the DKIM key's validity window has expired, even if the signature itself is mathematically valid.
3. Test the actual delivery path using MailTester’s inbox placement tester. This simulates a real-world email delivery from your domain to major providers and surfaces issues like expired keys that aren’t obvious from DNS checks alone.

Build resilience into your email infrastructure

4. Automate DKIM key rotation. Manually updating keys is error-prone and often leads to propagation delays. Set up a system that renews keys before expiry and updates DNS ahead of time.
5. Account for DNS propagation delays. Even after updating, DNS changes may take minutes to hours to propagate widely. Always deploy new keys before the old ones expire to avoid downtime.
6. Monitor your sender reputation and DKIM alignment. Use tools like RFC 6376 (which defines DKIM) to validate your implementation against industry standards, ensuring your infrastructure supports modern authentication practices.

Invalid or expired DKIM keys can break deliverability even when SPF and DMARC are set. Prevention is cheaper than recovery.

You don’t need to guess whether a key is expired. Tools like MailTester’s bulk verification service let you test multiple addresses with real-time feedback on authentication status. By identifying problematic keys before sending to a full list, you reduce bounces and improve inbox placement.

How MailTester integrates with existing systems to prevent issues

You can stop 451 4.7.5 errors caused by expired DKIM keys before they harm deliverability by using MailTester’s real-time API and integrations with platforms like Mailchimp, HubSpot, Klaviyo, and SendGrid. These tools automatically flag addresses with expired keys during onboarding or list cleanup, so only valid, well-configured emails ever reach your inbox. This proactive filtering reduces bounces, protects sender reputation, and keeps your campaigns in the inbox.

Real-time verification at scale

Let’s say you’re adding 10,000 new contacts a day. Instead of waiting for a bounce, MailTester’s API checks each email address in real time—before it gets into your campaign. This isn’t just a spam filter. It verifies DNS records, checks for catch-all aliases, identifies role accounts, and confirms whether DKIM signatures are currently valid. If the key has expired, you’ll know instantly. You can then remove or flag the address without sending.

Use the email verification API to build this step directly into your signup, CRM, or segmentation processes. It’s designed for developers who need reliable, scalable checks without managing infrastructure. The results are clear: valid, invalid, catch-all, or risky—each with a detailed explanation that helps you decide what to do next.

Seamless integration with your workflow

Most teams don’t verify emails in isolation. They’re managing campaigns through Mailchimp, sending nurture flows via HubSpot, or automating journeys with Klaviyo. MailTester plugs into these platforms to automatically scrub invalid or high-risk addresses before any email sends.

For example, when an email with an expired DKIM key enters a campaign via SendGrid, the integration blocks it based on real-time feedback from MailTester. This prevents hard bounces and avoids triggering spam filters. You’re not blocking emails arbitrarily—you’re preventing delivery issues caused by technical misconfigurations that many tools miss.

The in-app AI assistant can help you interpret results like 451 4.7.5, which indicates a temporary server issue—often a policy-based rejection when a domain’s DKIM key has expired. The error isn’t about the address being invalid, but the sender's setup being out of sync. MailTester explains this contextually, so you don't misdiagnose it as a bounce from a dead address.

Learn more about the types of email verification results and how they affect deliverability at our bulk verification page. It’s how teams keep their sender reputation intact while reducing wasted sends. For high-volume operations, this level of integration is not optional—it’s essential.

Why deliverability testing is essential for high-volume senders

You can’t rely on static email validation or even perfect DKIM signatures to guarantee inbox placement. Even with a valid DKIM key, domain policies, expired keys, or greylisting can trigger rejections like 451 4.7.5 — especially during bulk sends. Real-world testing across Gmail, Outlook, and Yahoo exposes these hidden roadblocks before they damage your sender reputation.

What bulk verification alone misses

• Static checks won’t catch domain-specific rejection policies that only appear under real delivery conditions.
• Expired or rotated DKIM keys still pass basic syntax checks but fail in practice — especially when your send volume triggers stricter monitoring.
• Greylisting and temporary rate limits only surface during live delivery — they’re invisible to list scrubbing tools.
• Even one 451 4.7.5 error from a major provider can trigger increased scrutiny, reducing your inbox placement over time.

How inbox placement testing works in practice

• Testing across real mailboxes (Gmail, Outlook, Yahoo) reveals which domains block or delay your messages — not just why, but how often.
• These tests reflect actual filtering rules, including spam-weighting, sender reputation thresholds, and authentication mismatches.
• MailTester’s inbox placement tool simulates real sends to multiple inboxes, showing failure rates and error codes like 451 4.7.5 under realistic load.
• Use inbox placement testing before major campaigns to catch policy rejections your list verification tools miss.
• Even with valid DNS records, your message may be blocked due to historical sender behavior or temporary delays — only active testing exposes this.

Think of it this way: a clean email list doesn’t mean clean delivery. An expired key or strict policy can silently block mail regardless of syntax correctness. DKIM ensures authenticity, but deliverability depends on the recipient’s policies — not just your setup.

You’re not alone: 451 4.7.5 with valid DKIM is a real, recurring issue

The 451 4.7.5 error with a valid DKIM signature is not a flaw in your setup. It reflects the ongoing, industry-wide shift toward stricter enforcement of key lifecycle management by receiving mail servers.

Large-scale senders in finance, e-commerce, and SaaS report similar issues during key rollovers, especially when keys are in transition or have been expired for a short period, even if the DKIM signature itself remains technically valid.

Prevent delivery failures with proactive verification

• Verify sender reputation and domain alignment before sending.
• Check for expired or rotating keys using real-time validation tools.
• Predict inbox placement issues before large campaigns launch.

Sources

• 52.1% of the world's top 1.8 million domains (937,931 domains) now publish a valid DMARC record, up from 29.1% in 2023. — EasyDMARC 2026 DMARC Adoption & Enforcement Report (2026)
• 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)
• DKIM b= Tag Fails Due to Carriage Returns in Signature
• Why Some DNS Providers Preserve Case but Receivers Normalize It in DKIM
• DNS TXT Record Case Sensitivity for DKIM d= Tag in 2026
• Why Does SPF Record Fail When Domain Uses CNAME for IP Lookup?

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

Frequently asked questions

What does 451 4.7.5 mean in the context of DKIM?

451 4.7.5 is an SMTP error indicating a policy rejection. It can occur even with valid DKIM if the public key is expired or no longer trusted by the receiving server.

Can DKIM be valid but still cause delivery failure?

Yes. A valid signature confirms message integrity and origin, but not key freshness. Expired keys trigger 451 4.7.5 in systems enforcing key lifecycle policies.

How does MailTester detect expired DKIM keys?

It checks the DNS TXT record for the DKIM public key and validates both the signature and the key’s expiration state if present.

Is 451 4.7.5 a temporary or permanent error?

It’s a permanent error. The sending server must correct the underlying issue—such as updating the expired key—before delivery can succeed.

Do all email providers reject messages with expired DKIM keys?

Not all, but many major providers enforce key lifecycle policies. Rejection rates vary, but failure to update keys increases the risk.

Can a catch-all mailbox cause a 451 4.7.5 error?

No. Catch-alls do not cause 451 4.7.5. That error is tied to policy, not mailbox status. It’s unrelated to whether an address exists.

How often should DKIM keys be rotated?

Standard practice is every 3–6 months. Rotation must include publishing the new key well before deactivating the old one.

How can I test if my DKIM key is expired?

Use MailTester’s inbox-placement tests or check DNS TXT records for the key’s expiration tag. Tools with real SMTP simulation provide the most accurate results.

Does MailTester work with bulk list verification for expired DKIM detection?

Yes. MailTester’s bulk list verification identifies high-risk addresses—including those with expired DKIM keys—before sending begins.

Can MailTester help with DMARC policy issues?

Yes. By simulating delivery and validating SPF, DKIM, and DMARC alignment, it helps identify misconfigurations that impact deliverability.