How to Debug DKIM Signature Expiry from Malformed t= Tag in Long-Lived Messages

Why does a malformed t= tag cause DKIM signature expiry in long-lived messages?

You send a transactional email today. It’s time-stamped with a DKIM signature. Weeks later, the same message lands in a user’s inbox—clean, valid, but suddenly marked as failed or delayed. Why? Because the t= tag in the DKIM signature isn’t a timestamp of when the signature was generated—no, it’s a *verification window* baked into the message itself. And if that window is miswritten, the receiver treats the message as expired, even if the content is valid. The t= tag must be a Unix timestamp in seconds, with no formatting, no extra characters, and within a sensible range. If it’s missing, set to a future date (like 2099), or includes non-numeric characters, the receiving server can’t validate the signature’s age. That triggers rejection, especially when the message is sent long after creation. This isn’t theory—it’s a common silent killer of long-lived transactional emails, backup newsletters, and archival alerts.

Key takeaways

• The t= tag in DKIM signatures defines a validity window based on when the signature was created, not when it expires.
• A malformed t= tag—such as a non-numeric value, missing timestamp, or future date—can cause receivers to reject the signature as expired, even for long-lived messages.
• Strict adherence to RFC 6376 is required: t= must be a clean, 10-digit Unix timestamp with no delimiters.

How do long-lived messages expose issues with the t= tag?

Messages signed with a fixed DKIM signature and a malformed or future-dated t= tag can fail validation months after sending if the timestamp is outside the allowed window. Since the t= tag defines the signature's validity period, an incorrect value means the signature appears expired during verification—even if the message is otherwise intact. Strict mail servers reject such messages, breaking deliverability for long-lived content like monthly newsletters or automated reminders.

Why the t= tag matters in time-sensitive validation

DKIM signatures include a timestamp parameter (t=) that tells receiving servers when the signature was created. If this timestamp is set incorrectly—say, using a future date or malformed format—it can be flagged as invalid when the server checks for time window compliance. Mail servers that enforce strict cryptographic validation, such as those used by financial or enterprise providers, will reject messages with out-of-window signatures.

Let’s say your newsletter sends a DKIM-signed message in January with a t=1700000000 (which is roughly December 2023), but you resend the same template in June with the same signature—no change, no regeneration. If the server’s validation window is only 14 days, and the signature’s t= is interpreted as being in the past and beyond the window, the message could be rejected. This is especially common with static templates or stored automated emails sent months apart.

How to catch t= issues before they break deliverability

When a DKIM signature uses a static t= value, it’s critical that the timestamp aligns with actual sending times. If you’re sending emails months apart, ensure your signing process either updates the t= value at time of send or uses a reasonable validity window. For long-lived messages, using a dynamic t= value—generated each time the message is sent—minimizes risk.

Even small syntax errors in the t= parameter (like a non-integer value or incorrect format) can lead to rejection. Valid timestamps must be Unix epoch seconds; using a human-readable date or a string instead will break parsing. The DKIM base specification explicitly defines how timestamp values should be processed during signature validation.

Testing your DKIM signature’s validity across time is difficult without tooling. You can use inbox placement testing to simulate real-world delivery and catch validation failures before they hit your audience. Regularly checking the full envelope, including headers and DKIM components, helps identify static or malformed signatures early.

What does a malformed t= tag look like in practice?

You'll see a malformed t= tag in DKIM signatures when the timestamp value is invalid—like using a non-numeric string, an invalid date format (e.g., February 30), or a timestamp with too many digits. These errors break the DKIM verification process and can cause long-lived messages to fail validation, even if the rest of the signature is correct.

Common mistakes in the t= tag

• Invalid date format: t=2026-02-30 — not a valid date. February doesn’t have 30 days. This will fail validation since the t= tag must be a Unix timestamp, not a human-readable date.
• Non-numeric characters: t=123abc — includes letters and symbols. DKIM requires a clean integer timestamp in seconds since the Unix epoch.
• Too many digits: t=17091234567 — 11 digits. The maximum valid Unix timestamp (for years before 2106) is 10 digits. This exceeds the limit, causing parsing failure.
• Correct format: t=1709123456 — a valid Unix timestamp (February 22, 2024). This is the expected format in RFC 6376, the standard governing DKIM.

Why this matters for long-lived messages

Messages with long validity periods (e.g., newsletters, transactional PDFs) often reuse DKIM signatures. If the t= tag is malformed, even a minor error invalidates the entire signature upon inspection. This leads to delivery failures, especially with modern mail providers like Gmail and Outlook that enforce strict DKIM validation.

According to RFC 6376, the t= tag must be a 10-digit integer representing seconds since January 1, 1970. Any deviation breaks compliance. This is why even small mistakes in timestamp generation can cause problems at scale.

Let’s say you’re sending a quarterly report with a DKIM signature set to expire in 2025. If the t= value is set to 17091234567 by a faulty script, the receiving server will reject the signature—despite the message being otherwise valid. The fix? Validate and sanitize all t= values before signing.

If you're checking DKIM validity or testing how messages will be received across inboxes, use tools that simulate real-world checks. MailTester’s inbox placement tests verify how your DKIM-signed emails perform across major providers—spotting issues like expired or malformed timestamps before they hit inboxes.

How to detect DKIM signature issues before they cause delivery failure

You can catch DKIM signature issues—like expired t= tags in long-lived messages—before they cause bounces or rejections by validating signatures in real time. Use a verification API to test individual messages before sending, monitor your DKIM signing process for hardcoded timestamps, and log t= values, especially for emails sent with long delays. This proactive approach avoids delivery failures due to expired or malformed DKIM signatures.

Test DKIM signatures in real time before sending

Let’s say you’re sending a campaign with time-sensitive content, like a scheduled newsletter or onboarding drip. If your DKIM signature’s t= tag isn’t updated per message, it may expire before delivery, even if the message is otherwise valid. You don’t want to wait for a bounce to find out. Use a real-time email verification API to validate the DKIM signature state of individual messages before they go out. MailTester’s API checks the full envelope, including DKIM validity, at the point of send.

Check your DKIM signing system for hard-coded timestamps

DKIM signatures include a timestamp, encoded in the t= tag, which defines how long a signature remains valid. If your email system uses a static t= value—say, set once per campaign—then all messages in that batch inherit the same expiration time. Over time, this can lead to expired signatures, especially if messages are delayed or sent hours later than intended. This is a common issue in systems that batch-sign messages without per-message timestamp updates.

For long-lived messages—like automated workflows or triggered emails sent hours or days after creation—this becomes a real risk. Even a simple misconfiguration in your signing process can trigger rejection. According to RFC 6376, a DKIM signature must have a valid t= tag that reflects a timestamp within the message’s lifetime. If the timestamp is too old or malformed, receivers will reject it.

Step-by-step: How to diagnose and fix malformed t= tags

You can fix DKIM signature expiry from malformed t= tags by extracting the raw message, validating the t= timestamp using a DKIM tool like dkimvalidator.com, ensuring it’s a positive 10-digit Unix timestamp not exceeding 2147483647 (January 19, 2038), and re-signing the message with a correct value. Automated inbox placement testing confirms the fix works.

Extract and validate the raw message

1. Log into your email delivery system (e.g. SendGrid, Amazon SES, or your own MTA) and retrieve a raw message in RFC 5322 format. This includes headers and body, unaltered by rendering tools.
2. Use a public tool like dkimvalidator.com to parse the DKIM-Signature header. It’ll decode the signature and expose individual tags, including t=.
3. Check if the t= value is a plain integer. A non-integer, negative number, or string (e.g., "123abc") means it’s malformed or corrupted.

Ensure the timestamp is valid and within bounds

4. Verify the t= value is a positive 10-digit number representing a Unix timestamp. For example, 1700000000 is valid; 0, -1, or 9999999999 are not.
5. Confirm the timestamp doesn’t exceed 2147483647—the maximum signed 32-bit integer. Going beyond this causes a 2038 bug, rendering the signature invalid in systems that enforce strict bounds.
6. If the signature is expired (e.g., timestamp in the past) or malformed, you must re-sign the message with a correct t= value—typically the current time.

Re-signing is often done in your mailer using DKIM signing libraries like dkimpy, which support programmatic signature generation. Ensure your signing process includes a dynamic t= value, not a hardcoded one.

After fixing, send a test message through an inbox placement service to confirm deliverability. Tools like MailTester’s inbox placement tester can simulate real inboxes and check DKIM validation outcomes across providers.

Never assume DKIM is working just because it’s set up. Many delivery failures stem from subtle issues like off-by-one timestamps or misformatted tags. Regular validation of signed messages prevents long-lived bugs from slipping into production.

How to test DKIM signatures at scale using MailTester

You can test DKIM signatures at scale with MailTester by using its real-time verification API or bulk verification tool to simulate delivery across hundreds or thousands of messages. The platform checks the full DKIM signature structure—including the t= tag—during a simulated send, flagging expired, malformed, or invalid signatures with immediate feedback. This lets you catch issues like expired t= tags in long-lived emails before they fail in production.

Test DKIM validity during real-time delivery simulation

When you send a message through MailTester’s verification process, it doesn’t just validate the address—it fully simulates the SMTP transaction, including DNS lookups, TLS negotiation, and DKIM signature parsing. This means the t= tag is checked against actual timestamp rules defined in RFC 6376, which require properly formatted timestamps. If the time value is malformed, out of range, or past its expiry, MailTester reports it directly.

Messages with invalid or expired t= tags are returned with a clear verdict, so you can identify and patch the root cause—whether it’s a misconfigured signing tool, a delay between signing and sending, or a flawed automation pipeline. The response includes a detailed breakdown of the signature components, helping you confirm whether the issue is the t= tag itself or another part of the DKIM structure.

Integrate with your email platform for continuous validation

MailTester works with your existing workflow. You can connect it directly to SendGrid, Mailchimp, or HubSpot via our integrations to verify DKIM signatures on messages sent through those platforms without manual intervention. This ensures every email processed through your system meets inbox-reliability standards before it’s delivered.

For high-volume senders, the API allows automated testing of DKIM signatures across your entire mailing list. You can run checks on archived campaigns, long-running email journeys, or transactional workflows where messages may be sent days after signing—making it ideal for catching outdated or malformed t= tags in delayed sequences.

DKIM is an industry-standard practice for email authentication, and proper signature validation ensures message integrity and sender reputation. As outlined in RFC 6376, the t= tag plays a key role in limiting the window during which a signature remains valid. Tools like MailTester help enforce that standard during mass testing. With 98.9% accuracy across real-world messages, MailTester offers a reliable, transparent way to find and fix DKIM issues before they impact deliverability.

Common causes of incorrect t= values in DKIM signatures

You're seeing DKIM signature expiry errors because the t= timestamp in your signed messages isn't updating per send. This usually happens when a legacy script reuses a hardcoded timestamp, a cron job misinterprets system time, or a database stores timestamps in a non-Unix format. When a message is resent after a long interval without regenerating the signature, the t= value becomes invalid and the signature fails validation. Let's break down the actual root causes.

Static or misconfigured timestamp sources

• Using a static t= value in a legacy signing script that doesn't update for each delivery. This is common in systems not designed for dynamic signing.
• Misconfiguring a cron job or scheduler to use local time instead of UTC, or to read system time from a misaligned clock, leading to invalid or outdated timestamps.
• Parsing timestamps from a database that stores values in a non-Unix format (like ISO 8601 with time zones or milliseconds), which can result in incorrect t= values when converted incorrectly.

Forgotten signature regeneration after message resends

• Resending a message after a long period (e.g., in a retry queue or after bounce recovery) without regenerating the DKIM signature, causing the original t= value to exceed the validity window.
• Assuming that a signed message can be safely resent multiple times without re-signing, especially in systems that store messages for later delivery or batch processing.
• Not properly validating that the t= value is within the acceptable range (typically 120 seconds after i=), which can lead to rejection by receiving servers.

When testing email deliverability, ensure that your DKIM signature is regenerated for every send — including resends — and that the t= value reflects the actual time of signing in Unix epoch format. The DKIM RFC defines the t= tag as a required timestamp indicating when the signature was created; an expired or incorrect value will cause the signature to fail validation.

For developers rebuilding legacy systems, verify that your signing pipeline respects the timing constraints of DKIM. You can validate your output by using tools like MailTester's inbox placement test to see how your signed messages are processed in real inboxes — including DKIM validation outcomes. This helps catch issues before they impact sender reputation.

How sender reputation is impacted by expired or malformed DKIM signatures

Expired or malformed DKIM signatures—especially those with invalid t= tags—trigger repeated validation failures. Even a single failed signature can degrade sender reputation over time, leading to increased spam filtering, lower inbox placement, and eventual delivery throttling. Receiving servers monitor long-term authentication consistency; persistent failures signal unreliability, making your domain appear suspicious even if the content is legitimate.

Consistent DKIM failures erode trust with receiving servers

When DKIM signatures fail due to malformed t= parameters, mail servers log those failures. High volumes of failed validations over time feed into sender reputation systems used by major providers like Gmail and Outlook. These systems don’t just look at one message—they track patterns across your domain's entire sending history. A pattern of repeated signature failures is treated as a red flag, even if only one message in a long-lived campaign is affected.

Let’s say you send a transactional email with a t= tag set to a future date that has since passed. If the receiving server checks the t= timestamp at delivery, it sees an expired or invalid value. This triggers a DKIM validation failure, even though your public key is correct and message integrity is fine. The server records it. The longer these failures accumulate, the more likely your domain is to be flagged as a potential source of abuse.

Single failures can lead to broader deliverability consequences

Even one invalid DKIM signature in a long-lived message—such as a newsletter or campaign that’s sent over weeks—can be reported to blocklists if it’s flagged by multiple recipient servers. If this happens frequently, some blocklists may apply temporary or even permanent filters to your domain IP or domain, regardless of other legitimate mail.

Reputation systems are designed to penalize patterns of failure, not individual messages. A few failed validations are usually ignored. But a consistent pattern—especially one linked to a structural flaw like an incorrect t= tag—can lead to filtering on all messages from your domain, not just the malformed ones. This is why fixing the root cause matters: it prevents cascading deliverability issues.

For example, RFC 6376 defines the correct format for the t= tag, which must be a Unix timestamp in seconds. If your signing tool sets it too far in the future, or uses an invalid value like t=1234567890x, the signature will fail validation on the receiving end. These kinds of errors are subtle but impactful.

Before sending to large lists, verify your DKIM setup and test messages with tools that simulate real-mail server behavior. Use inbox placement testing to check whether your messages pass authentication checks under realistic conditions—and whether any malformed headers are affecting delivery.

Best practices to avoid malformed t= tags in DKIM signatures

Always set the t= tag in your DKIM signature to the current Unix timestamp at signing time. Validate it as a 10-digit integer before sending—no exceptions. Never use random or static values, and never generate t= from a stored or derived timestamp. Automate checks at every send, especially for long-lived campaigns where timing drifts can cause expiry issues.

Signature generation and validation

• Generate the t= tag using the exact Unix timestamp at the moment your message is signed—no rounding, no caching, no pre-calculation.
• Validate that t= is a 10-digit integer before sending. A malformed value like 16409952000 (11 digits) or abc breaks verification.
• Use a time-based nonce or randomizer only for other DKIM header fields (like b= hash variations), not for t=. The t= tag is time-based by design, not randomness.
• Never reuse a t= value across messages or batches—each signed message must have a unique time-stamp.

Integration and automation

• Integrate DKIM verification checks into every outbound message pipeline—especially for long-lived campaigns where messages may stay in queues or be resent after delays.
• Use automated tools to scan generated DKIM signatures in real time. A single malformed t= tag can cause rejection by receiving mail servers, even if the rest of the signature is valid.
• Test with industry-standard tools like MXToolbox’s DKIM checker to validate output before sending to production.
• Monitor your email deliverability using platforms like Return Path, which track DKIM signature issues at scale across domains and providers.

The t= tag is not a security or anti-spoofing feature—it’s a timestamp. If it’s inaccurate, expired, or malformed, the signature fails validation. Even a single incorrect digit can lead to a rejected message or reduced sender reputation.

Sending reliably begins not with the message body, but with correct metadata—in particular, a valid, real-time t= tag in DKIM.

For teams managing large-scale email campaigns, automated testing helps catch these errors before they impact deliverability. Consider running full DKIM signature checks on every batch using tools like the MailTester Inbox Placement tool, which simulates real-world receiving behavior including verification of all signature fields.

How MailTester helps prevent delivery failures from signature issues

You can catch DKIM signature issues like malformed t= tags early by testing your messages in real-world conditions. MailTester’s inbox placement testing simulates how ISPs verify DKIM during delivery, including checking the t= timestamp for validity. If the timestamp is missing, malformed, or expired in long-lived messages, MailTester flags it immediately, so you fix it before sending.

Real-time diagnostics reveal signature flaws before they break delivery

When you use the MailTester real-time verification API, you get clear, actionable feedback on each email’s DKIM health. If the t= tag is improperly formatted—like a missing timestamp, a non-numeric value, or a future date—the API returns a structured diagnostic. This isn’t just a “valid” or “invalid” response; it tells you exactly what’s wrong, so you can adjust your signing process.

Bulk checks keep large lists clean and deliverable

For campaigns with thousands of recipients, catching one malformed DKIM signature can be enough to trigger delivery scrutiny. MailTester’s bulk verification tool scans your entire list, identifying addresses with signature issues across multiple messages. It’s not just about email format—your DKIM configuration applies to each individual message. If your system signs messages with an expired t= tag, MailTester detects it consistently across your list, helping prevent mass delivery failures.

MailTester’s 98.9% accuracy ensures you’re not chasing false positives. Since credits never expire, you can run periodic audits—on top of every campaign—to maintain long-term visibility into your sender health. Unlike some tools that charge per test or limit data retention, MailTester keeps your inbox placement confidence stable over time.

For deeper insight into how ISPs evaluate your messages, you can use the inbox placement tester to see how your DKIM signature holds up in live environments across providers like Gmail and Outlook. It’s the closest you’ll get to real-world feedback without sending to real users. See how it works: run a full inbox placement simulation.

DNS and signing setup are hard to debug in isolation. But by testing your messages under realistic conditions—including DKIM timestamp validation—you reduce the risk of getting filtered or blocked. Tools like this, combined with good practices like adhering to the DKIM RFC, make email delivery more predictable. If your system relies on long-lived DKIM signatures, make sure the t= tag is properly generated and within a valid range.

Conclusion: Fix the root cause, not just the symptoms

A malformed t= tag in a DKIM signature isn’t a minor parsing glitch—it breaks authentication, triggers bounces, and harms sender reputation over time.

Check your email infrastructure: ensure time values in DKIM signatures are correctly formatted and updated on every send. A single mistake in a t= value can invalidate the entire signature.

Use tools like MailTester to test signatures before sending. Real-time verification and inbox-placement testing catch these issues early, before they impact real users or trigger blocklists.

Sources

• DMARC adoption among top domains surged 75% between 2023 and 2025 — from 27.2% to 47.7% — in the wake of Google and Yahoo's bulk-sender authentication requirements. — EasyDMARC 2025 DMARC Adoption Report (2025)
• After Gmail began requiring authentication for large senders, the number of unauthenticated messages Gmail users received plummeted by 75%. — Google (The Keyword blog) (2023)

Keep reading

• Email authentication: SPF, DKIM, DMARC, BIMI and MTA-STS (complete guide)
• Impact of Delayed DNS Resolution on DMARC Aggregate Report Collection
• How Oversized SPF Records Cause DNS Response Truncation and Verification Delays
• Configuring SPF, DKIM, and DMARC for Lemlist and Apollo Success
• Email Verification API That Detects DKIM Errors from Case-Insensitive Header Processing

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

Frequently asked questions

What happens if the t= tag in a DKIM signature is missing?

Mail servers may reject the signature if they require a timestamp, especially for long-lived messages. Some servers may accept it, but a missing t= tag reduces trust and can impact deliverability.

Can a DKIM signature be valid if the t= tag is in the future?

No. If the t= timestamp is set to a future time, receivers may treat the signature as expired or invalid due to time-window checks, especially when the message is delivered after the timestamp.

How long is a DKIM signature valid?

DKIM signatures don’t have a set expiration. However, if the t= tag is too old or set incorrectly, servers may flag the signature as invalid. Validation is time-sensitive based on the t= value.

Does MailTester detect issues with the t= tag in DKIM signatures?

Yes. The real-time verification API and inbox placement testing evaluate DKIM signature structure, including the t= tag, for validity and compliance with RFC 6376.

Is a malformed t= tag a sign of a larger email infrastructure problem?

Yes. Frequent or repeated malformed t= tags indicate flaws in the signing process. It often reflects poor automation, lack of validation, or outdated signing logic.

Can a message be delivered with a malformed DKIM signature?

Some servers may deliver the message, but others will reject or flag it. The result is inconsistent delivery, higher bounce rates, and damage to sender reputation.

How do I test if my email infrastructure is signing correctly?

Use MailTester’s API to send test messages through real delivery paths. It validates DKIM structure, including t= tag format, during inbox placement testing.

What is the difference between t= and x= in DKIM?

The t= tag signals the creation time of the signature (Unix timestamp). The x= tag, if present, defines the expiration time. If both are used, expiration must be after the signature time.

Do all mail servers check the t= tag in DKIM signatures?

Most major servers do. While not all enforce time validation, many do—especially those with high spam filtering standards. A malformed t= tag increases the risk of rejection.

Can I fix a malformed t= tag after the message is sent?

No. Once sent, the signature cannot be corrected. The only way forward is to resend the message with a valid t= tag and ensure the signing process is fixed.

How does MailTester integrate with SendGrid and Mailchimp?

MailTester integrates with SendGrid and Mailchimp via direct API connections. You can verify DKIM signatures and inbox placement as part of workflows in those platforms.

Do I need to use a specific DKIM tool to debug t= issues?

No. MailTester’s inbox placement testing simulates real-world validation, including t= tag checks. You don’t need third-party tools if you use reliable verification.