Fix t= microsecond timestamp error

What causes an email security verification error due to t= microsecond timestamp?

You sent an email that was marked as invalid — not because of a typo, but because a timestamp in the DKIM signature included microseconds. This can happen even when everything else is correct. You didn’t make a mistake. The system did.

This is not a rare glitch. It happens when the 't=' field in a DKIM signature exceeds 10 digits, which breaks strict validation rules. Most modern systems accept it, but some legacy or security-hardened environments reject it outright. The error isn’t about your email client or server — it’s about how timestamps are processed during security verification.

Key takeaways

The 't=' field in DKIM signatures uses Unix epoch time; values with more than 10 digits (i.e. including microseconds) are rejected by strict validators.
Microsecond timestamps (e.g. t=1700000000123456) are valid under RFC 6376 but may fail in environments that enforce rigid 10-digit limits.
This issue arises in compliance-heavy or legacy systems, not due to flawed email setup, but due to overly strict interpretation of DKIM standards.

Why does t= microsecond timestamp cause verification failure?

DKIM signatures require the t= field to be a 10-digit Unix timestamp in seconds since January 1, 1970. If microseconds are included (14 digits), the signature fails validation because it violates RFC 6376. Even if the time is correct, strict validators reject the signature outright, leading to a security error during email verification.

What RFC 6376 says about t=

According to RFC 6376, the t= tag must be a 10-digit integer representing the number of seconds since the Unix epoch. This specification is unambiguous: microseconds are not allowed. Including them—say, t=1712345678901 instead of t=1712345678—means the timestamp is technically invalid, even if the underlying time is precise.

Some filtering systems use strict regex patterns to parse DKIM headers. When they encounter a 14-digit value where a 10-digit one is expected, they reject the entire signature. This isn’t a flaw in the timestamp value—it’s a flaw in the implementation. The server sees a violation of format rules and blocks the email, even if the signature otherwise checks out.

Why this matters during verification

When you're verifying email addresses at scale, a single malformed DKIM signature can cause a delivery failure you won’t catch with basic syntax checks. This often appears as a vague "security verification error" in logs, making root cause hard to track without digging into header parsing.

Tools like MailTester help you catch these issues before they hit your inbox. Our email list verification service checks not just syntax, but actual deliverability signs—including DKIM signature validity, SPF alignment, and DMARC policy enforcement. You can run a bulk verification to identify problematic domains or senders before your campaign goes live.

Run a bulk verification to detect formatting errors like misaligned timestamps, and ensure your emails pass technical checks before sending.

How to detect t= microsecond timestamp issues in DKIM signatures?

You can detect t= microsecond timestamp issues by analyzing the raw DKIM signature in email headers using a DKIM analyzer tool. Look for the t= tag with 13 or more digits—any value beyond 10 digits (like t=1700000000123456) indicates a timestamp in microseconds, which violates RFC 6376. This non-compliance breaks DKIM validation, leading to failed authentication and email delivery issues.

Use a DKIM analyzer tool to examine raw email headers

Open the full email header of a message that failed delivery or was marked as suspicious.
Find the DKIM-Signature header line and extract the full value, including all tags like t=, d=, and s=.
Paste the DKIM-Signature string into a free, trusted DKIM analyzer like DKIM Validator or MXToolbox DKIM Check to parse and validate the signature components.
These tools will break down the signature and highlight any non-standard fields, including invalid timestamps.

Look for t= values with 13+ digits

Check the t= value specifically. A valid DKIM timestamp must be a 10-digit Unix epoch.
Values like t=1700000000 (10 digits) are valid. Values like t=1700000000123 (13 digits) are invalid and indicate a microsecond-level timestamp.
Common red flags include t=1700000000999999 or similar—these suggest a system that mistakenly added microseconds instead of seconds.
Such errors typically originate from misconfigured email platforms or outdated libraries that incorrectly format timestamps during signing.
Once identified, fix the underlying software or API that generates the signature—ensure it only uses second-level epoch values.

DKIM signing tools must use exactly 10-digit Unix timestamps; any deviation breaks signature validation, even if the rest of the signature is correct.

While no email-verification tool directly flags t= microsecond issues—because they're part of the signature, not the address itself—verifying your email infrastructure with a proper DKIM analyzer is step one. For teams sending at scale, use bulk list verification to catch invalid domains or problematic senders early, and pair it with header analysis to isolate delivery failures caused by cryptographic errors.

Step-by-step: How to fix a t= microsecond timestamp error

If your email fails DKIM verification due to a t= parameter with more than 10 digits, the issue is a timestamp with microseconds. DKIM RFC 6376 requires the t= value to be a 10-digit Unix timestamp in seconds. You must truncate or round microseconds before signing. Use your email system’s raw headers to find the signature, adjust the timestamp, re-sign, and resend. This ensures compliance with industry standards.

Locate and inspect the DKIM signature

Open the problematic email in a mail client that shows raw headers, or use a tool like MxToolbox to retrieve the full email source.
Look for the DKIM-Signature: header line. It will include parameters like v=1, a=rsa-sha256, and importantly, t=.
Copy the entire DKIM-Signature field. Paste it into a text editor to examine it clearly — the t= value should be a 10-digit number, representing seconds since the Unix epoch.

Fix the timestamp and re-sign

Check the length of the t= value. If it has more than 10 digits, it includes microseconds. For example, t=1712345678901 is invalid — it includes 3 extra digits.
Strip the microseconds. Take only the first 10 digits (the seconds). For instance, 1712345678901 becomes 1712345678. This is the valid timestamp format.
Modify your email signing process so future messages use only full seconds when generating the t= parameter. Don’t send a timestamp with fractional seconds — even if your system supports it, DKIM strictly enforces this.
Re-sign the email with the corrected timestamp. Ensure your signing library or email service applies the truncated time before hashing.
Send the email again. Check the new DKIM signature using tools like DKIM Validator or MxToolbox to confirm the t= value is now a valid 10-digit number.

Even if your system auto-generates timestamps, verify it doesn’t prepend microseconds. Many platforms default to microseconds for precision — but DKIM doesn’t accept them. This error is commonly caused by misconfigured libraries or custom signing scripts that fail to truncate. A single invalid timestamp can result in outright rejection by receiving servers.

Once fixed, regular email sends should pass DKIM validation. If you're managing large volumes, use MailTester’s email checker to validate individual addresses before sending, and inbox placement testing to observe real-world delivery. This helps catch signature issues before they impact your sender reputation.

Can you prevent t= microsecond timestamp issues before sending?

Yes — you can prevent t= timestamp errors by validating DKIM signatures during email generation, before they’re sent. These errors occur when the 't=' timestamp in a DKIM signature exceeds 10 digits, which violates RFC 6376. Catching this early avoids rejected messages and protects sender reputation.

Validate DKIM signatures before sending

DKIM signing is a critical step in email authentication, but it’s easy to introduce errors if the timestamp isn’t formatted correctly. The 't=' tag must contain exactly 10 digits representing seconds since Unix epoch. A timestamp with microsecond precision (e.g., 1700000000123) breaks this rule and causes verification failures. You can catch this before sending by validating the signature format as part of your email workflow.

Tools that check 't=' length, SPF/DKIM alignment, and DNS consistency during message generation help catch these issues early. This is especially valuable when building automated systems or campaigns that rely on dynamic content and variable headers.

Integrate verification during development and campaign setup

Let’s be clear: you don’t need to wait for bounces or blocks to find problems. By integrating a real-time email verification system like MailTester during development and campaign setup, you can catch malformed DKIM signatures — including t= issues — before they hit the inbox.

MailTester’s verification API checks not just syntax, but also the underlying authentication practices. You can validate domains, verify DNS records, and audit signature structure, including the 't=' timestamp format. This kind of pre-send validation is standard practice in regulated industries and high-volume senders where inbox placement matters.

For developers, using the real-time verification API during testing phases ensures every outbound email meets technical standards. For marketing teams, the bulk list verification tool screens entire campaigns for risk factors, including broken authentication, well before delivery.

DKIM alignment with SPF and DMARC, and correct timestamp handling, are not optional. They are part of email security best practice. Misconfigured signatures reduce deliverability and hurt trust. Catching the 't=' microsecond error early is a small step with real impact.

For reference, the RFC 6376 outlines the precise structure of DKIM signatures. While it doesn’t mandate 't=' length, it specifies the format clearly — and any deviation can cause rejection. Tools that validate against this standard are not just helpful. They’re necessary.

How MailTester helps prevent t= microsecond timestamp errors

MailTester catches t= microsecond timestamp errors by testing your emails against live mail servers before you send. It doesn’t generate DKIM signatures, but it detects delivery failures caused by invalid 't=' values during inbox-placement tests—helping you avoid bounces and spam filtering caused by malformed cryptographic timestamps.

Testing Real Delivery, Not Just Syntax

Most tools only check if an email address is syntactically valid or if it accepts mail. MailTester goes further: it simulates actual sends to real mail servers. This means it surfaces issues like malformed DKIM signatures—including invalid 't=' timestamps—that wouldn’t show up in basic syntax checks.

Digital signatures like DKIM rely on precise timing values. If the 't=' timestamp is in microseconds instead of seconds, many mail servers reject the message outright. According to RFC 6376 (the DKIM standard), timestamp values are expected in seconds since the Unix epoch, not microseconds. A mismatch here is a common trigger for delivery failure, even with valid domains and IPs.

Let’s say your system accidentally includes a timestamp in microseconds. MailTester’s inbox-placement test will catch that when it attempts to deliver your message to real mail servers like Gmail, Yahoo, or Outlook. If the message fails due to a malformed 't=' value, you’ll know before your campaign goes live. No guesswork. No wasted sends.

Real-Time and Bulk Checks for Proactive Security

Whether you're validating one address with the email checker or scrubbing a list of 50,000 with the bulk verification, MailTester checks for infrastructure-level issues. It doesn’t just confirm "this address exists"—it checks whether it will actually reach the inbox, or fail silently due to technical errors like timestamp mismatches.

You can also integrate MailTester into your workflow via the real-time verification API, validating addresses on sign-up or during sending. This prevents invalid signatures from ever hitting your mail queue.

It’s not about avoiding every failure—it’s about catching the subtle ones that ruin your sender reputation. Invalid 't=' timestamps may not affect all recipients, but they can trigger automated filters. MailTester surfaces these before you send, so you know what’s truly deliverable—and what’s broken under the hood.

The risk of sending emails with invalid DKIM 't=' timestamps

Even a single incorrect microsecond in a DKIM signature's 't=' timestamp can trigger rejection by high-security filters, especially in government or enterprise environments. These systems enforce strict cryptographic validation — a one-millisecond deviation is enough to invalidate a signature, leading to messages being flagged as spam, quarantined, or silently dropped. This isn’t just a minor glitch; it directly impacts deliverability and erodes sender reputation over time.

Why small timing errors have big consequences

DKIM uses a 't=' timestamp to verify when a signature was created, ensuring messages aren’t replayed or delayed maliciously. RFC 6376 (the standard governing DKIM) requires this timestamp to be within a reasonable window — typically up to 300 seconds, but real-world validators often apply stricter rules. When the timestamp is off by even a few microseconds due to system clock drift or incorrect time formatting, the signature fails validation.

Corporate and government mail systems, which often use hardened filtering engines like Microsoft Defender or Cisco email security appliances, may reject such messages outright. Unlike a bounce, you won’t always get feedback — the email simply disappears into quarantine or is ignored. This invisibility makes detection hard, and repeated failures compound the damage by signaling instability or poor infrastructure to email providers.

Let’s say your campaign sends 10,000 emails with a flawed DKIM timestamp. Even if only 1% are rejected, that’s 100 messages failing silently. Over time, ISPs like Gmail and Outlook may start lowering your domain’s reputation score. That’s not just about delivery — it’s about long-term trust.

How to catch and fix it before it spreads

DKIM issues like this aren’t caught by basic email validation tools. You need a check that looks at the full cryptographic signature, not just the address format. That’s where real-time email verification with DKIM parsing comes in. Tools that analyze the actual email header and signature can flag timestamps outside expected ranges.

For example, MailTester’s bulk verification checks entire campaign lists for DKIM-related red flags, including malformed or invalid timestamps, before you send. This isn’t a one-off test — it’s part of a systematic defense against delivery failure. You’re not just checking if an address exists; you’re ensuring the entire email stack validates correctly.

For automated workflows, MailTester’s verification API includes DKIM validation as part of its 98.9% accurate checks, catching timing issues before they impact your domain. And because credits never expire, you can test consistently without risk of wasted spend.

Timekeeping in email systems isn’t just about clocks — it’s about trust. A timestamp off by microseconds can break that trust. Verify the details, and keep your messages from vanishing into the void.

How to test if your email system is vulnerable

Send a test email from your system to Gmail, Outlook, Yahoo, and a corporate domain. Check the full email header in each inbox for DKIM: fail or DKIM: none. If there’s no Authentication-Results header explaining the failure, your system may be including an invalid or malformed t= timestamp—often in microseconds—causing verification errors. Fixing this usually means adjusting your DKIM signing process to use standard second-level timestamps.

Check your DKIM signature in real-world headers

Send a test message from your sending system to a Gmail, Outlook, and Yahoo account. Use real user addresses, not test domains.
After delivery, open the message and show the full email header (in Gmail: three dots → "Show original"; in Outlook: "View message source").
Look for a DKIM-Signature: header line containing t=—it should have a Unix timestamp in seconds, not microseconds.
If you see Dkim: fail or Dkim: none with no Authentication-Results: header, the receiving server could not verify your signature—possibly due to an improperly formatted t= field.
If Authentication-Results exists, check if it mentions dkim=fail (invalid timestamp) or invalid timestamp (microsecond precision)—this points directly to the issue.

Analyze with third-party tools

Use Spamhaus or MxToolbox to analyze the full email chain from receipt to validation.
Input your full raw message header into their tools to check DKIM alignment and identify timestamp anomalies.
These tools can detect if your t= value exceeds 10 digits (indicating microseconds) or doesn’t match the standard time zone or format.
Per RFC 6376, DKIM timestamps must be in seconds since epoch, not microseconds. Microsecond precision is non-compliant and triggers failures in production systems.
You can also simulate sending through MailTester’s inbox placement tool to check real-world delivery outcomes and see how receivers interpret your DKIM signature.

Is this error common among email campaigns?

This error is rare but appears more often in automated systems that generate timestamps with microsecond precision—especially in high-frequency email flows, API-driven platforms, and enterprise signing libraries. It’s not a typical issue for consumer email clients, but it can disrupt campaigns in regulated industries where strict cryptographic standards apply. If you're seeing it, your email signing process is likely generating a timestamp with higher resolution than expected.

Why it shows up in automated systems

Some automated email platforms and signing libraries default to UTC timestamps with microsecond resolution. When a system includes a t= timestamp in a DKIM signature, and that timestamp exceeds the expected 32-bit second range (as defined in RFC 5322), it triggers a verification error. This isn’t a flaw in your email—it’s a mismatch between a strict timestamp format and a validation logic that assumes 32-bit seconds.

Let’s say your system generates a timestamp like t=1700000000.123456. If the receiving server expects only the integer seconds part and doesn’t handle microseconds, it may reject the signature—especially if it checks for valid time ranges or bounds. This is more common in compliance-heavy environments like finance or healthcare, where signature validity must be rigorously enforced.

Where it matters most

While you won’t see this in most consumer mail apps, it’s been reported in enterprise deployments using automated signing tools like those in Salesforce, SendGrid, or enterprise-grade email gateways. The issue becomes visible when DKIM checks fail in mail logs, leading to deliverability issues or messages marked as unverified.

For context, the RFC 5322 standard defines message syntax, including time formats, but leaves timestamp precision interpretation to implementers. This lack of strict enforcement means some servers handle microseconds gracefully, while others don’t. That variability can create silent failures in email flows.

If you're building automated campaigns, it's worth validating your email signing process isn’t injecting overly precise timestamps. You can test for this by checking your DKIM output or using a tool like MailTester’s email checker to validate the integrity of your message headers before sending.

Best practices to avoid t= microsecond timestamp issues

Always truncate or round DKIM timestamps to seconds before signing—never include microseconds. RFC 6376 explicitly requires the t= timestamp to be in seconds since Unix epoch, and including microseconds causes verification failure, even if other fields are correct. This is a common source of silent delivery issues that can break authentication without clear error messages.

Use compliance-aware tools and libraries

Choose DKIM signing libraries or email gateways that explicitly adhere to RFC 6376 and automatically normalize timestamps to seconds.
Verify your email service provider does not append microseconds by default—some platforms default to higher precision unintentionally.
Test DKIM signatures in a sandbox environment using real-world tools like MXToolbox or DMARCian to validate signature syntax before sending at scale.

Validate before deploying to production

Test all outbound messages through a live infrastructure setup—simulate real delivery paths with known receivers to catch subtle issues like microsecond timestamp mismatches.
Include DKIM validation as part of your pre-send checks. Use tools that parse and validate DKIM signatures, not just send them.
Use MailTester’s inbox placement tester to validate end-to-end deliverability, including signature compliance, before sending to your full list.

Even a single microsecond can break DKIM. Consistency in timestamp format is not optional—it’s required by the standard.

Let’s be clear: this isn’t a rare edge case. It’s a repeatable, fixable issue that affects every system that signs mail without normalizing timestamps. You can’t rely on email clients to forgive a malformed t= value—most will reject it outright. The fix is simple: always round timestamps to the nearest second. Tools that do it for you reduce risk. So do pre-send validation steps.

For teams managing large mail campaigns, automated verification helps avoid this entirely. You can use MailTester’s bulk verification to scrub lists before sending, catching invalid or misconfigured addresses early. If you’re building your own email flow, use the real-time verification API to validate individual addresses and their associated headers during outbound processing.

Bottom line: treat timestamp normalization as part of your SMTP stack. It’s not an afterthought. It’s part of the core security contract your emails must uphold.

What to do if you’re still seeing verification errors after fixing 't='

Fixing the t= microsecond timestamp error is a step, not a guarantee. If errors persist, the root issue may lie deeper in your email authentication stack.

Review SPF, DKIM, and DMARC alignment

One misconfigured record can override or mask more subtle problems. Verify that all three are properly set and aligned with your sending domain.

Ensure the domain in the From header matches the DKIM selector’s domain. A mismatch here will fail verification even if the timestamp is correct.

Simulate real inbox conditions

Authentication is only half the story. Even well-configured emails can land in spam or fail delivery due to sender reputation, content filtering, or recipient policies.

Use MailTester’s inbox-placement testing to simulate real-world receipt across providers. This confirms whether your messages actually reach inboxes or are blocked silently.

Keep reading

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Frequently asked questions

What does 't= microsecond timestamp' mean in DKIM?

It refers to a DKIM 't=' timestamp that includes microseconds (14 digits), violating RFC 6376’s rule that the field must be exactly 10 digits (seconds only).

Why does a t= timestamp with microseconds cause a verification error?

Some email servers enforce strict parsing rules. A 14-digit timestamp is outside the standard 10-digit range, so it fails validation even if the time is correct.

Can MailTester detect t= microsecond timestamp errors?

No, not directly — MailTester doesn’t analyze DKIM signatures. But its inbox-placement testing can reveal delivery failures caused by such issues.

How do I fix t= microsecond timestamp issues in my email system?

Update your signing logic to use only full seconds for the 't=' parameter, truncating or rounding microseconds before generating the DKIM signature.

Is this error a security risk?

No — it’s a compatibility issue, not a vulnerability. It causes delivery failure, not data exposure or spoofing.

Do all email servers reject t= microseconds?

No — only systems using strict DKIM parsers. Many mainstream providers accept signatures with microseconds, but enterprise systems often enforce strict rules.

How can I prevent this issue in future campaigns?

Validate DKIM signatures during development, use compliant signing libraries, and test messages with inbox-placement tools before sending to large lists.

Does MailTester integrate with my email platform?

Yes — MailTester integrates with Mailchimp, HubSpot, Klaviyo, and SendGrid to verify and clean lists before sending, improving deliverability and reducing errors.

Can I test for DKIM compliance with MailTester?

Not directly. But its deliverability testing simulates delivery to real inboxes and can surface issues like DKIM failures caused by strict validators.

What if my DKIM signature passes but messages still fail?

Check the authentication results, sender reputation, and message content. Use MailTester to rule out invalid addresses and other deliverability risks.

Is the t= microsecond error fixable after the message is sent?

No — if the message failed, you must re-sign, re-send, and ensure the new signature uses only seconds in the 't=' field.

Does MailTester guarantee zero deliverability failures?

No. MailTester improves reliability with 98.9% accuracy but cannot eliminate all delivery issues. Factors like blacklists, content filtering, and recipient behavior also play a role.