What happens when DKIM t= uses microseconds instead of seconds?

You send an email with a perfectly valid DKIM signature—everything checks out. Yet it bounces. Not because of syntax errors or bad DNS records. Because the timestamp in the DKIM header says t=1700000000000 instead of t=1700000000. A single extra digit, and your message fails authentication. This isn’t a flaw in your system—it’s a parsing error that some receivers catch, and others ignore.

DKIM’s t= field is supposed to be the Unix epoch time in seconds. But if your signing tool (or a custom parser) uses microseconds instead, receiving servers that strictly enforce RFC 6376 may reject the email. It’s a subtle mismatch, but one that breaks authentication even when all other checks pass.

Key takeaways

  • DKIM t= timestamps must be in seconds since the Unix epoch, not microseconds.
  • Receiving servers that enforce RFC 6376 strictly may reject messages with microsecond timestamps, even if all other authentication checks pass.
  • Legacy or custom DKIM parsers are more likely to misinterpret microseconds as seconds, leading to signature verification failures.

Why does the DKIM t= timestamp format matter for deliverability?

If a DKIM t= timestamp is in microseconds instead of seconds, strict validators reject the signature as malformed. Even one failed DKIM check can trigger spam filters, hurt sender reputation, or cause outright rejection. This is because DKIM validation is a gatekeeper for authentication — receivers don’t accept messages with any signature flaw, regardless of how minor.

How DKIM validation works in practice

When an email arrives, receivers verify the DKIM signature by checking the public key in DNS and re-computing the hash. The t= timestamp in the signature must match the time the email was sent, expressed in seconds since the Unix epoch. If it’s in microseconds — even if just one digit off — the validator flags it as invalid. This isn’t a quirk; it’s specified in RFC 6376, the standard for DKIM.

A single failed validation can seem small, but in practice, it’s a red flag. Major providers like Gmail and Microsoft rely on strict validation. If your domain consistently fails DKIM checks, even once, it can trigger filtering or cause your IP to be deprioritized. This isn’t about one message — it’s about systemic trust. A misformatted t= value breaks authentication, and broken authentication breaks deliverability.

How to verify and prevent timestamp errors

Let’s be clear: this isn’t a rare edge case. It can happen in automated systems that use system clocks or libraries that default to microseconds. You might think, "It’s just a number," but the consequence is real: undeliverable mail, lower sender reputation, and blocked domains.

Using tools like MailTester’s email verification API or inbox placement tester helps catch these issues before you send. The bulk verification tool scans your list for suspicious patterns, including malformed DKIM signatures. You can also test individual addresses with the email checker or integrate directly with your sending platform via the real-time verification API to validate messages on the fly.

For deeper insight, refer to RFC 6376, Section 3.3, which defines the required timestamp format. It states that the t= field must be in seconds, not microseconds. When your system generates signatures, ensure it’s using seconds. If you're using a library or framework, check its default behavior — some auto-convert timestamps into microseconds.

How microsecond timestamps in DKIM t= break validation

DKIM’s t= tag must use seconds, not microseconds. A value like 1700000000 is valid (Unix timestamp in seconds); 1700000000000 is not, even if it’s a correct microsecond timestamp. Some systems fail to parse t= correctly when it exceeds 10 digits, leading to missing or invalid timestamps that break signature validation.

The standard is clear: t= in seconds only

DKIM, as defined in RFC 6376, specifies that the t= tag must represent the timestamp in seconds since the Unix epoch. This is non-negotiable and widely enforced by mail servers and validation tools. If a system receives a t= value in microseconds—such as 1700000000000—it should reject the signature, but many don’t. Instead, they silently fail or misinterpret the value.

Let’s be clear: 1,700,000,000 seconds is a timestamp from around 2023. But 1,700,000,000,000 microseconds is an entirely different beast—approximately 54,000 years in the future. No legitimate DKIM signature should have a t= value that high. Yet some tools, especially misconfigured or older ones, may attempt to parse t= values as if they were microseconds, leading to false positives or validation failures.

Why your validator might still fail

Validation systems that expect seconds will silently skip a t= field if the value has more than 10 digits. They may not log a failure, leaving you with a "valid" signature that’s actually unusable. The signature checks pass, but the timestamp is either ignored or treated as invalid—either way, it undermines trust in the DKIM mechanism.

If you're troubleshooting delivery issues or inconsistent inbox placement, check your DKIM signing process. Even if your public key and domain are correct, an incorrect t= value—like one that uses microseconds—can be flagged as invalid by strict validators, especially in automated systems used for bulk email verification.

Use a real-time email verification tool to catch these issues early. The MailTester email checker validates full email technical correctness—including DKIM syntax, SPF alignment, and DNS records—before you send, helping prevent failures due to hidden parsing errors.

Common places where t= timestamp errors occur

DKIM t= timestamp errors most often crop up in legacy systems, custom signing scripts with flawed epoch logic, and testing environments using overly precise mock timestamps. The t= value must be in seconds since the Unix epoch, not microseconds — a common oversight when developers forget to divide by 1000. This leads to validation failures, even when all other DKIM fields are correct.

Legacy systems and outdated email platforms

  • Old email infrastructure from the early 2000s often used non-standard DKIM implementations where timestamp handling was inconsistent or undefined.
  • Some enterprise platforms from that era still rely on custom or patched DKIM signers not aligned with current specifications.
  • When migrating inbound mail from outdated systems, these misaligned t= values can cause legitimate emails to be rejected by modern receivers.

Custom signing scripts and test environments

  • Custom signing logic that generates timestamps in microseconds (e.g., from JavaScript's Date.now()) without dividing by 1000 results in values millions of times larger than expected.
  • Developers testing DKIM signature generation often use simulated epochs with high precision (e.g., nanosecond-level timestamps) to catch edge cases, but forget to normalize back to seconds during actual signing.
  • Automated testing frameworks sometimes inject mocked timestamps with fractional precision, which pass local checks but fail in production due to strict parsing rules.

Timestamp parsing in DKIM is defined in RFC 6376, Section 5.3, which explicitly states that the t= value must be a 32-bit integer representing seconds since the Unix epoch. Any value outside the 1970–2038 range or with incorrect units violates the standard, even if the rest of the signature is valid.

Let’s be clear: a single misplaced decimal can cause an email to be rejected. This is especially true when verifying high-volume lists, where a single invalid signature can trigger sender reputation issues.

When using MailTester’s bulk email verification, you can detect DKIM-related validation issues early — including malformed t= values — before they degrade sender reputation or cause delivery failures.

How to detect DKIM t= timestamp format issues in real email traffic

DKIM t= timestamp issues often appear as signature verification failures without clear sender policy problems. You’ll see them when the t= value in the DKIM signature has 12 or 13 digits—indicating microseconds instead of seconds—common in some poorly implemented or flawed signing tools. Use tools that parse raw headers and validate the full DKIM chain to catch these subtle format errors before they impact deliverability.

Check for subtle signature verification failures

  • Scan your email logs for "DKIM signature verification failed" alerts, especially when SPF and DKIM alignment are otherwise correct.
  • Look for messages where the t= value in the DKIM signature header has 12 or 13 digits—this is a strong signal of a microsecond timestamp being used instead of seconds.
  • Compare t= values across multiple messages from the same domain: inconsistent or unusually large values (e.g., 1672531200000) often point to incorrect timestamp formatting.
  • Verify that the t= value falls within a reasonable range—typically within a few days of the current time—to rule out clock drift or misencoded timestamps.

Validate signatures with a full-stack tool

  • Use an email verification or analytics tool that performs end-to-end DKIM signature validation, including proper parsing of the entire header block.
  • Ensure the tool checks the RFC 6376-compliant timestamp format, where t= must represent seconds since Unix epoch, not microseconds.
  • Test a sample of your outbound emails using a service like MailTester’s inbox placement tool, which includes DKIM and header validation as part of its deliverability scan.
  • For high-volume senders, integrate a real-time API such as MailTester’s verification API to validate sender alignment and timing fields in real time before sending.
  • Reference RFC 6376 for the official specification—especially the t= field definition—to confirm that your tools and systems comply with the standard.

Use real-time DKIM verification to catch parsing errors before sending

Senders who fail to validate DKIM signatures—including timestamp format—risk rejection or delay. MailTester’s real-time verification API checks DKIM signatures in real time, including the t= timestamp, flagging values with microsecond precision as risky or invalid. This catches problems before they harm sender reputation.

Prevent delivery failures with real-time DKIM checks

When a DKIM signature uses a t= timestamp in microseconds instead of seconds, it’s non-compliant with the standard. While some servers may accept it, many will reject it outright or treat it as suspicious. This breaks authentication and can lead to bounces, especially with strict filters from Gmail, Microsoft, or enterprise gateways.

Let’s be clear: the DKIM RFC (RFC 6376) specifies the t= timestamp in seconds since the Unix epoch. Using microseconds violates that specification. MailTester detects this deviation immediately—no waiting for delivery failures or post-facto reports from blocklists.

Verify before sending, at scale

You can test individual messages or bulk lists via the verification API to catch malformed t= values before they get sent. This includes edge cases like timestamps that are too large, too small, or formatted with microsecond precision.

Running checks during list hygiene or before campaign sends ensures that only compliant and deliverable messages proceed. This proactive step reduces hard bounces and protects your sender reputation. The cost of a single failed DKIM signature—especially when multiplied across thousands of emails—can be high in terms of deliverability and inbox placement.

For teams already using platforms like SendGrid, Klaviyo, or HubSpot, integrating MailTester's API into the workflow means these issues are caught mid-pipeline, not after the sending stage. This is standard in high-volume email operations where even rare failures disrupt performance.

While not every email gateway enforces the t= format strictly, ignoring non-standard values increases the risk of being labeled as low-quality sender. As RFC 6376 confirms, timestamps must be in seconds. Following the standard is both a technical imperative and a credibility signal.

How MailTester helps validate DKIM signature compliance

You can catch DKIM timestamp errors before they hurt your deliverability by validating that the t= timestamp in your DKIM signatures is in seconds, not microseconds. MailTester checks the t= value during real-time and bulk verification, flagging signatures with microseconds or values exceeding the maximum allowed seconds. This stops issues that cause inbox placement failures before they happen.

Why t= microseconds break DKIM alignment

DKIM specifies that the t= timestamp must be in seconds since the Unix epoch. If your email system or third-party service outputs microseconds instead, it violates the standard. This misalignment is ignored by some mail servers, but others reject messages outright or flag them as suspicious. Even if the signature is cryptographically valid, a malformed t= value can cause routing or authentication failures.

MailTester parses DKIM headers and checks the t= value against the expected format—integer seconds, not fractional or microsecond precision. If it sees 1700000000 instead of 1700000000, it returns a "risky" or "invalid" verdict. This includes cases where a timestamp exceeds the 32-bit Unix epoch limit (which is 2,147,483,647 seconds), a known issue in legacy systems.

Because DKIM verification is part of the overall email authentication stack, a single error can undermine SPF and DMARC alignment. That’s why catching t= issues early matters. The most common source is automated tools or libraries that default to microseconds—especially in newer development stacks like Python’s time.time() or Node.js timestamps without conversion.

Fix issues before they hit your sender reputation

Let’s say your marketing platform signs emails with a t= value in microseconds. Without validation, this goes unnoticed until a customer doesn’t receive a campaign. By then, your sender reputation may already be declining. MailTester identifies these errors during bulk verification or real-time checks, so you can fix your signing process before rolling out messages at scale.

Use bulk verification to audit large lists, or integrate our API into your send logic to catch invalid signatures before delivery. The results clearly label whether a signature is valid, invalid, or risky based on DKIM header compliance—especially timestamp format—and include actionable details.

For deeper validation, see inbox placement testing to simulate how your messages land across inboxes. This combines DKIM, SPF, DMARC, and content checks. For more details on the technical standard, refer to the DKIM spec (RFC 6376, section 3.2), which defines the t= requirement.

How to fix a DKIM t= timestamp error when your system uses microseconds

If your DKIM signature includes a t= timestamp in microseconds instead of seconds, it will fail validation. The t= value must be the number of seconds since the Unix epoch. If your system generates timestamps in microseconds, divide by 1000 before inserting into the signature. This is a common issue when using high-precision time functions without conversion.

Step-by-step fix

  1. Locate the DKIM signature generation code — find where the t= tag is inserted in your email signing process. This could be in a library, module, or custom script. Many modern systems report time in microseconds; ensure your code is not passing raw timestamps directly.
  2. Convert microseconds to seconds — if the timestamp is in microseconds, divide it by 1000 before using it in the DKIM header. For example, a value of 1700000000000 microseconds becomes 1700000000 seconds. This aligns with the DKIM specification, which requires seconds.
  3. Validate the output format — test that the t= value in your signed email appears as an integer with no decimal points. Any fractional part or non-integer format will trigger validation failure. This is required by the RFC 6376.
  4. Send a test message through MailTester — use the inbox placement tester to send a sample email with the corrected signature. The tool checks real-world validation across major inboxes and confirms whether DKIM passes.

Common causes and prevention

Modern JavaScript and some systems use Date.now() or performance.now(), which return microseconds. These must be adjusted before DKIM application. Libraries that handle signing should not assume time units — always verify the output.

Always test with real inbox simulators. A signature may pass basic syntax checks but still fail in production if the timestamp is misaligned. Tools like Spamhaus and MxToolbox help diagnose issues, but they don’t test the full delivery path.

Best practices for DKIM signature generation to avoid t= issues

Always generate DKIM signatures using standard libraries that follow RFC 6376, ensuring the t= timestamp is in seconds (not microseconds). A common parsing error occurs when systems use microsecond timestamps, which mail servers interpret as invalid. This breaks DKIM validation and can trigger delivery failures. Let’s get it right from the start.

Use standard libraries and validate against the RFC

  • Don’t roll your own DKIM signer. Use well-tested libraries like OpenDKIM, libdmarc, or the built-in tools in popular mail stacks (e.g., Postfix with OpenDKIM).
  • Confirm that all generated t= values are in seconds, not microseconds. The RFC requires this format — any deviation will cause validation to fail.
  • Before sending, verify your DKIM headers against RFC 6376, especially the timestamp and signature format. Automated linting tools can help check compliance.

Test in real-world conditions

  • Integrate DKIM signature validation into your testing pipeline using tools that simulate actual mail server behavior.
  • Use inbox placement tests to verify that emails with your DKIM headers actually reach inboxes and pass authentication checks.
  • Run bulk checks with MailTester's bulk verification if you’re sending to large lists — it can surface issues like malformed DKIM headers across multiple recipients.
  • Test with public DKIM validators (e.g., MxToolbox's DKIM tool) to double-check your generated signatures.
One misplaced microsecond in a t= timestamp can break email reputation and deliverability — even if everything else is correct.

MailTester’s role in preventing DKIM parsing problems at scale

You can catch DKIM timestamp errors—like t= values in microseconds instead of seconds—before they hurt deliverability by scanning large lists with MailTester’s bulk verification tool. It flags inconsistent or malformed DKIM signatures across domains, helping you fix parsing issues at scale before sending.

Spotting infrastructure patterns early

When you verify hundreds or thousands of addresses at once, MailTester reveals structural flaws in your email infrastructure. A recurring t= timestamp issue—where microsecond values aren’t standardized—can indicate flawed email signing logic, often tied to misconfigured mail servers or outdated libraries. These patterns appear across domains, signaling a broader setup issue, not just isolated bad addresses.

MailTester’s bulk verification doesn’t just check syntax; it identifies clusters where DKIM signatures fail due to inconsistent timestamp formatting. These red flags often stem from systems that use system_clock() or similar low-level functions without proper rounding or unit conversion. RFC 6376 (the DKIM standard) requires the timestamp to be in seconds since the Unix epoch, not microseconds. Tools like RFC 6376 specify that precision beyond seconds is not valid for t=.

Real-time validation and automated fixes

During onboarding or automation flows, the MailTester real-time API can intercept addresses with suspicious DKIM signatures before they ever hit the send queue. If a system returns a risky verdict due to a malformed t= value, you can reject or flag it instantly. This prevents bad emails from ever being sent, protecting sender reputation.

Combine that with inbox placement testing to confirm the fix works. By simulating delivery to Gmail, Outlook, and Yahoo, you can verify whether corrected DKIM signatures improve inbox placement. If an address was previously rejected due to DKIM parsing errors—especially those caused by microsecond timestamps—the test shows whether it now arrives reliably.

Use the bulk list verification tool to audit your database, the real-time API to harden workflows, and the inbox placement tester to validate results. You’re not guessing anymore—you’re acting on data, not assumptions.

You don’t need to guess — trust in verified results

DKIM t= timestamp errors in microseconds instead of seconds are subtle, often invisible in standard delivery logs. They don’t trigger immediate bounces, but they do cause signature validation failures, which degrade sender reputation over time.

Even if emails reach the inbox, inconsistent or malformed DKIM signatures lead to filtering, reduced engagement, and long-term deliverability issues. These problems compound silently until they impact campaign performance or trigger blacklisting.

MailTester’s 98.9% accuracy catches these issues before they affect your list, ensuring only valid, properly signed emails are sent. You’re not relying on guesswork — you’re acting on verified, real-time data.

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Frequently asked questions

What does DKIM t= timestamp mean?

The t= tag in a DKIM signature specifies the time the signature was created, in seconds since the Unix epoch. It is used to validate the signature's age and prevent replay attacks.

Why does t= in microseconds break DKIM validation?

Receiving servers expect t= in seconds, not microseconds. A value like 1700000000000 is interpreted as 1.7 trillion seconds since 1970 — far beyond any valid time period — and fails validation.

Can a t= timestamp with 13 digits still be valid?

No, if it's in microseconds. Valid timestamps in seconds are at most 10 digits. Values over 10 digits are likely mistaken for microseconds and will cause parsing errors.

Does MailTester detect DKIM t= timestamp formatting issues?

Yes, MailTester checks DKIM headers during verification and flags t= values that are in microseconds or outside the valid range as risky or invalid.

How often do t= timestamp parsing errors occur?

They’re uncommon in well-maintained systems but can appear in older platforms, custom implementations, or during testing with debug timestamps.

Can I fix t= errors after an email is sent?

No. Once sent, you cannot fix the DKIM signature. Fix the signing process and monitor future messages for the same issue.

Which email systems are most likely to reject malformed t= values?

Email servers from major providers like Google, Microsoft, and Apple enforce DKIM strictly. They reject messages with invalid or malformed t= values.

How does Microseconds affect sender reputation?

Repeated failed DKIM verifications, even due to timestamps, reduce sender reputation and can lead to increased spam filtering or blocklist placement.

Does MailTester test inbox placement for DKIM errors?

Yes. MailTester’s inbox placement testing includes DKIM signature validation to confirm messages are not blocked due to formatting issues.

What’s the difference between SPF, DKIM, and DMARC?

SPF validates the sending server’s IP address. DKIM signs the message content. DMARC enforces policies based on SPF and DKIM results. All three are needed for strong authentication.