How to Fix DKIM Public Key Not Recognized Due to Malformed TXT

Why is your DKIM public key not recognized by receiving servers?

You sent a carefully crafted email. The DKIM signature passed validation in your test tool. But the recipient’s server rejects it with a cryptic error: “public key not recognized.” You check your domain, your DNS settings, your email client—everything looks correct. Yet deliverability remains broken.

The issue likely isn’t your email content or the DKIM key’s cryptographic integrity. It’s how the public key appears in your DNS TXT record. Even a single typo, missing space, or incorrect line wrapping can render a valid key unusable. Receiving servers don’t interpret keys like humans do—they parse DNS records exactly as they’re written.

Understanding how DKIM keys are stored and validated—especially the mechanics behind the “public key not recognized” error—is essential for fixing deliverability issues that seem to come out of nowhere. This guide walks through the exact causes, common mistakes, and how to verify and fix your DNS TXT record with confidence.

Key takeaways

• DKIM public keys must be stored in DNS TXT records with exact formatting—any deviation breaks validation.
• Even a single misplaced character, such as an incorrect hyphen or missing space, can cause the “public key not recognized” error.
• Keys must be properly wrapped in quotes and not broken across lines unless strictly following DNS line length rules (max 255 characters per segment).

What does 'DKIM public key not recognized' actually mean?

When a receiving server says your DKIM public key is not recognized, it means the server found your DNS TXT record but couldn’t read the key due to formatting errors—like missing quotes, broken line breaks, or a key that’s too long. This breaks the DKIM signature validation, which can cause your emails to be flagged as suspicious or rejected, even if they're legitimate.

Why the key fails to parse

DKIM keys are often long strings that must be wrapped in quotes in DNS TXT records. Omitting those quotes, especially around keys over 255 characters, is a common mistake. Some DNS providers auto-split long entries, but without proper formatting, the key becomes unreadable. Even small syntax issues—like extra spaces or incorrect character encoding—can prevent the server from recognizing your key.

According to RFC 6376, the DKIM specification, public keys must be formatted as a single, quoted string within a TXT record. If the key is split across multiple records or not enclosed properly, the receiving server treats it as invalid. This isn’t about the key being wrong—it’s about it being unreadable due to technical format issues.

Why this matters for deliverability

When DKIM fails, the receiving server can’t verify that your message hasn’t been tampered with. Without a valid signature, your email may be treated as untrusted. This leads to higher spam filtering, worse sender reputation, and lower inbox placement—especially with platforms like Gmail, Yahoo, and Outlook that enforce strict authentication checks.

Even if SPF and DMARC pass, a failed DKIM check can still trigger delivery issues. It’s one of the most common authentication pitfalls that go unnoticed until deliverability drops. Many organizations only discover the issue after emails start bouncing or landing in spam folders.

Regularly auditing your DNS records, especially for long keys, helps prevent this. You can use tools like MxToolbox or DKIM Analyzer to verify format correctness. If you're managing large lists, a bulk email verification tool can help you catch misconfigured domains early—before they impact your sender reputation.

How DKIM DNS records work: the mechanics behind the failure

DKIM fails to recognize your public key if the TXT record contains line breaks or spaces after newlines, because DNS treats each line as a separate string fragment. The public key must be a single unbroken string following the p= parameter in the record. Even a single extra space or newline breaks the key, rendering it invalid regardless of correctness. This is not a server-side issue—it's a formatting error in how the TXT record is stored.

Understanding the DKIM TXT record structure

When you set up DKIM, you create a DNS TXT record with a name like selector._domainkey.example.com. The value must start with v=DKIM1; k=rsa; p=, followed by the entire public key as a single line. Any breaks—whether intentional or from a copy-paste glitch—break the key parsing.

Let’s say you paste the key across multiple lines in your DNS provider’s interface. The system stores each line separately, so instead of one continuous key, the TXT record becomes multiple strings. When the receiving server reads it, it concatenates all fragments—resulting in an invalid key. This is why tools like RFC 6376 specify that the p= value must be a single, uninterrupted sequence.

Why line breaks cause parsing failure

DNS servers do not interpret line breaks in TXT records. They treat them as separate string components. If your key appears in your DNS as:

p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...

and the system splits it across lines like:

p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA...

the receiver sees two strings. Even if they’re reassembled correctly, the key structure is corrupted—often by a single added or missing character. This leads to a "public key not recognized" error, even if the key itself is mathematically valid.

Misformatted records are common, especially with legacy tools or manual edits. Many DNS providers display TXT records as multiple lines for readability, but the underlying storage must be a single string. Always verify that the full key appears as one continuous line in DNS.

Use a DNS lookup tool like MXToolbox to inspect your record and ensure no truncation or fragmentation occurs. If you're uncertain, verify your DKIM setup with a service like MailTester’s inbox placement test, which checks not just delivery but alignment and signature integrity.

Step-by-step: diagnose and fix malformed DKIM TXT records

You’re getting "DKIM public key not recognized" errors because your DNS TXT record contains invisible characters, line breaks, or improperly formatted key data. Fix it by checking the full TXT value in a tool that reveals hidden characters, ensuring it starts with v=DKIM1; k=rsa; p=, and that the public key is a single base64 string with no spaces or quotes. Once cleaned, re-publish and verify via DNS lookup tools.

Diagnose the issue with visibility

1. You need to check the actual TXT record value as it’s stored in DNS. Log in to your DNS provider—Cloudflare, AWS Route 53, GoDaddy, or another—and locate the DKIM TXT record for your domain. This record must contain the public key used to validate signed emails.
2. Copy the entire value, including any quoted parts, and paste it into a text editor that shows invisible characters, such as VS Code (with "Render Whitespace" enabled) or Notepad++. This reveals line breaks, trailing spaces, or embedded quotes that aren't visible in DNS dashboards.
3. Ensure the value starts with v=DKIM1; k=rsa; p=. The v and k tags are required. If they’re missing, the record is invalid. After p=, the public key must follow as one continuous base64 string without line breaks, spaces, or quotes.

Fix and verify the record

4. Remove all line breaks, spaces after line breaks, and any surrounding quotes around the key. The entire string after p= must be a single uninterrupted base64 sequence. Even one extra space can break DKIM validation.
5. Save the corrected record and re-publish it in your DNS provider. DNS changes can take time to propagate—up to 48 hours, but often faster.
6. Use a real-time DNS lookup tool like Google’s DNS or dig from your terminal to verify the record resolves correctly. Run dig txt yourdomain.com and check that the output shows the exact, cleaned string.
7. Confirm the key is valid by testing it in a signature validator tool. The DKIM signature must be able to verify using the published public key. RFC 6376 (the standard for DKIM) specifies the required format and syntax.

If you're unsure whether a domain’s DKIM setup is working, you can verify its DNS records and email deliverability with MailTester’s real-time email checker. It will validate not only the email format but also DNS records tied to deliverability, including DKIM, SPF, and DMARC.

How to test if your DKIM TXT record is valid and properly formatted

You can confirm your DKIM TXT record is valid by querying it directly through a DNS checker like MXToolbox or Google’s DNS Lookup. Paste the full record value exactly as it appears in your DNS provider, then verify that the tool returns one clean result with no truncation or splitting. If you see multiple values, a partial key, or a message about record length, your DKIM TXT is likely malformed or split across entries beyond the 255-character limit per DNS entry.

Use a reliable DNS checker to validate your record

• Go to MXToolbox or use Google’s DNS Lookup to query your domain and DKIM selector (e.g., default._domainkey.example.com).
• Ensure you’re querying the exact selector and domain name used in your DKIM setup — case-sensitive and spelled correctly.
• Paste the full TXT record value (including quotes if present) directly into the tool’s input field.

Confirm the record is properly formatted and not split

• Check that the DNS tool returns only one record. Multiple entries suggest the key was split incorrectly across multiple TXT records.
• Verify the raw value does not exceed 255 characters. DNS TXT records are limited to 255 characters per entry; longer values must be split into multiple records with proper quoting.
• If the tool shows “partially displayed” or “truncated,” your record is split or incorrectly formatted — recheck your DNS provider’s entry.
• Use RFC 6376 Section 3.6 to confirm the correct syntax: the key must be a single string, enclosed in quotes if including spaces or special characters.

Let’s be clear: a malformed DKIM TXT record won’t be recognized by receiving servers, even if the key itself is technically correct. This breaks email authentication and leads to delivery failures or spam filtering. Double-check every character, especially the v=DKIM1; tag at the start and the p= parameter. If you're using a bulk email service, consider validating your entire list with MailTester’s bulk verification to catch delivery issues early.

Common formatting mistakes that break DKIM validation

DKIM fails when the public key isn't recognized because it’s stored with line breaks inside the p= value, wrapped in quotes, or split across multiple TXT records. Even tiny formatting errors—like extra spaces, tab characters, or non-printable ASCII—can prevent the receiving server from parsing the key correctly. You can test your DNS configuration with tools like MXToolbox or consult the DKIM specification (RFC 6376) to ensure your record follows the standard.

Line breaks inside the p= value

Even if your DNS dashboard shows a clean, single line, some providers store TXT records as multiple strings if you insert line breaks. This breaks DKIM validation because the receiving server expects a single, continuous value for p=. For example, inserting a carriage return or newline within the public key string makes it appear as two separate values in DNS, which the verifier treats as an incomplete or invalid key.

Quoting the entire key or using invalid characters

Adding quotes around the full p= value—like "v=DKIM1; k=rsa; p=..."—is not allowed by the DKIM standard. The p= value must be a bare string without quotation marks. Including any non-ASCII character, tab, or trailing space in the key also causes parsing errors. These characters might look invisible in your DNS editor but still break the verification process.

Another common issue is splitting the key across multiple TXT records without proper merging. Unlike SPF records, which can be combined across multiple TXT entries, DKIM requires the complete p= value to exist in a single record. If your DNS provider stores it across multiple entries, the receiving server sees only fragments and rejects the signature.

Let’s be clear: the DKIM spec is strict. A single mistyped space can mean your email fails authentication. Use verified tools to test your configuration before sending. If you're unsure, check your key’s syntax with our real-time email checker, which validates full DKIM setup in context with deliverability.

Why manual DNS verification is error-prone — and how MailTester helps

Manually checking TXT records for DKIM, SPF, and DMARC across domains and selectors is slow, complex, and prone to errors. A single misplaced quote, missing space, or truncated key can break email authentication and harm deliverability. MailTester’s real-time verification API checks all three protocols in one step, validates the full DNS chain, and flags malformed syntax immediately—saving hours and preventing bounces.

Manual DNS checks miss subtle but critical issues

When you’re checking DKIM records by hand, you’re looking for a TXT record with a specific selector and value. But even if the record exists, it might have a syntax flaw: a missing quote, a broken line break, or a key that’s been cut off. These aren’t always visible in standard DNS lookup tools, and they’re easy to miss under pressure. According to RFC 6376—the standard for DKIM—invalid or malformed keys result in authentication failure, which can lead to spam filtering or outright rejection by receiving servers.

Let’s say you’re setting up DKIM with a selector like default. The record must be exactly: default._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=...". If you forget the quotes or misplace a space, the key fails validation. The receiver sees it as malformed and drops the email—no warning, no explanation. Manual checks often stop at "does it exist?" and miss the deeper issues that impact inbox placement.

MailTester validates the full chain, not just the record

With MailTester’s real-time verification API, you don’t just check if a TXT record is present—you validate its content, format, and alignment across DNS resolution, key syntax, and selector matching. It checks whether the key is truncated (common with long RSA keys), properly quoted, and correctly formatted per spec. The tool identifies issues like improperly encoded public keys, missing or extra spaces, and invalid characters that would otherwise go unnoticed.

Unlike basic DNS tools, MailTester doesn’t just return raw data—it analyzes the result against known standards. This is why MailTester achieves 98.9% accuracy: it’s built to catch the kinds of edge cases that trip up manual verification. If you’re using tools like SendGrid, HubSpot, or Klaviyo, or integrating with Mailchimp, this kind of validation prevents sending failures before they happen.

For teams managing multiple domains or daily mailing campaigns, this isn’t just a time-saver—it’s a deliverability safeguard. Try it with a bulk list using MailTester’s bulk verification tool, or integrate the API for real-time checks during signup flows. You can also test full inbox placement before sending at scale with MailTester’s inbox tester. All results are transparent, precise, and actionable—from syntax errors to full authentication chain failures.

How to verify DKIM configuration at scale with MailTester

You can check DKIM records across multiple domains or senders in minutes using MailTester’s bulk verification API. Upload a list of domains and selectors—regardless of platform—and get real-time DNS validation, including detection of malformed TXT records. The in-app AI assistant identifies syntax errors and suggests fixes, while integrations with SendGrid, Mailchimp, and Klaviyo embed DKIM checks into your delivery workflow.

Step-by-step: Validate DKIM at scale with real-time feedback

1. Prepare your DKIM data — Gather a list of domains and their associated selectors (e.g., default._domainkey.example.com). This can include senders from different ESPs like SendGrid or Klaviyo. You don’t need to know the underlying setup—just the domain and selector.
2. Upload via the bulk verification API — Use MailTester’s bulk verification tool to upload your list. The API processes up to 100,000 entries in a single batch. It queries DNS records directly and returns the status of each DKIM TXT record in real time.
3. Review results and errors — You’ll see clear feedback: valid, invalid, or malformed. Malformed entries often stem from missing quotes, incorrect syntax (e.g., unescaped spaces), or overly long values exceeding DNS limits. For example, v=DKIM1; k=rsa; p=... must be enclosed in quotes when using special characters.
4. Use the AI assistant to decode errors — If a record fails, the in-app AI explains the root issue—such as "missing quotes around the public key" or "value exceeds 255 characters." It suggests exact syntax corrections, helping you fix configurations before they cause delivery failures.
5. Integrate with your email platform — Connect MailTester to SendGrid, Mailchimp, or Klaviyo via the integrations dashboard. This enables pre-send DKIM validation during campaign setup, reducing the risk of failed authentication.

Making it part of your workflow

DKIM isn’t a one-time setup. As domains change or new senders are added, verification should happen routinely. By using the API, you can run nightly checks on all active domains. This is especially useful after migrations or when scaling sends across teams.

For context, RFC 6376 specifies the correct format for DKIM TXT records. When misconfigured, they can lead to failed authentication and inbox placement drops—meaningfully more bounces. Tools like MXToolbox can confirm basic DNS syntax, but only MailTester’s real-time API gives you bulk validation with error guidance. You can test the health of your entire email infrastructure before it impacts deliverability.

“Proactive DKIM checks reduce the risk of messages being marked as spam by up to 40% in high-volume environments.”

DKIM, SPF, and DMARC: their distinct roles and how they work together

You need all three—SPF, DKIM, and DMARC—configured properly to ensure your emails land in inboxes, not spam folders. SPF checks if the sending IP is authorized, DKIM cryptographically verifies message integrity, and DMARC sets policies for what happens when either fails. A misconfigured DKIM record, like a malformed TXT entry, breaks the chain and leads to delivery failures, even if SPF is correct. It’s not enough to have one; all must align.

How Each Protocol Works in Practice

Let’s break down what each standard actually does.

For example, if your DKIM public key is not recognized due to a malformed TXT record, the receiving server will see a failed signature — even if SPF passes. DMARC policies then take effect, and if set to reject, your email may be blocked entirely. RFC 6376 defines DKIM’s technical requirements, including correct DNS formatting.

Even a single typo in a DKIM TXT record — like a missing space or an extra hyphen — can cause validation to fail. Use a tool like MailTester’s email checker to test whether your DKIM records are readable and correctly structured before sending.

Running a bulk list through MailTester’s bulk verification helps catch these issues early. The service checks for common DNS errors, including malformed DKIM records, while also validating deliverability, catch-all addresses, and disposable domains. It’s a complete diagnostic for your sending infrastructure.

When SPF, DKIM, and DMARC are all correctly aligned, you build sender reputation. When one fails, the entire system fractures. It’s not optional — it’s how email authentication works.

Once fixed, how to verify your DKIM is working in real-world send scenarios

After correcting your DKIM public key DNS entry, send a real email from your domain using a standard client like Gmail or Outlook. Check the full message headers for a valid DKIM-Signature with a=rsa-sha256 and a non-empty b= value. If the receiving server shows DKIM verification: pass, your setup is working. For real-world validation, use inbox placement testing to confirm delivery to major providers like Gmail, Yahoo, and Microsoft.

Check DKIM in real message headers

• Send a test email from your verified domain using a standard email client (Gmail, Outlook, Apple Mail).
• Open the full message headers—on Gmail, click the three-dot menu and select "Show original"; on Outlook, right-click the email and choose "View source."
• Look for the DKIM-Signature: header. It must include a=rsa-sha256—this specifies the hashing and signing algorithm used.
• Confirm that the b= tag contains a base64-encoded signature, not empty or malformed. A missing or zero-length b= means the signature failed during sending.
• If the validation status is pass (or equivalent in the receiver's report), the receiving server trusts your DKIM setup and your message is likely to avoid filtering.

Validate delivery in real inboxes

• Use MailTester’s inbox placement testing to send actual messages to real Gmail, Yahoo, and Microsoft inboxes.
• Cover a range of receivers across different provider ecosystems—some will use stricter algorithms than others, especially for new or low-reputation senders.
• Review the report for delivery status and delivery time. A delay or rejection often points to alignment issues beyond DKIM (like SPF or reputation).
• Check if the DKIM signature is still present in the final inbox message. Some providers strip or alter headers during delivery—this test proves whether your signature survives end-to-end.
• For a deeper validation, send using your actual sending platform (SendGrid, Mailchimp, etc.) and compare results. If the issue only appears in your test, the real-world setup is likely sound.

DKIM validation is not just about the DNS record—it's about consistent behavior across the send path. A correctly published key that fails to sign messages properly will still be rejected.

For developers and ops teams, this layered testing ensures that your infrastructure, not just the DNS record, is aligned with industry standards. The RFC 6376 defines DKIM’s core mechanics—including header formatting and signature structure—which all providers use. You’re not just verifying the key; you’re validating the complete signing workflow.

If you're validating multiple addresses across a list, use MailTester’s bulk verification to flag invalid or risky addresses before sending. This reduces the risk of triggering feedback loops or reputation damage due to poor sender hygiene.

Conclusion: don’t assume your DKIM is working — verify it

DKIM failures from malformed TXT records are common and often overlooked. Even one incorrectly formatted record can block emails, damage sender reputation, and reduce inbox placement.

Fix the syntax, not the theory

DKIM public keys must be a single, unbroken line of base64 text in the DNS TXT record. No line breaks, no quotes, no extra spaces. A single misplaced character breaks verification.

Automate detection across your domains

Manually checking each record is error-prone and time-consuming. Tools like MailTester scan your DNS configurations at scale, flagging malformed entries and helping prevent deliverability issues before they impact your campaign performance.

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)
• 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)
• Preventing DKIM Errors Caused by System Clock Misalignment
• Why DKIM Selector Not Resolved When TTL Is Negative in DNS
• Domain Mismatch in DKIM and SPF Causes DMARC Failures
• How to Prevent DKIM Signature Conflicts When Using Multiple Domains

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

Frequently asked questions

What is a malformed DKIM TXT record?

A malformed DKIM TXT record contains syntax errors like line breaks within the public key, extra quotes, or multiple fragmented strings, which prevent receivers from parsing the key correctly.

Can a DKIM error be caused by my email service provider?

Yes, some ESPs generate misformatted keys or split the record across multiple TXT entries. Confirm the full key is published as a single string in DNS.

How do I know if my DKIM record is split across multiple entries?

Check your DNS zone file for multiple TXT records with the same selector. If they exist, combine them into one single entry with no line breaks or extra punctuation.

Does the DKIM public key need to be wrapped in quotes?

No — the 'p=' value in the TXT record must be a plain base64 string. Quotes around the key are invalid and will break DKIM verification.

Why does my DNS checker show the DKIM record but MailTester says it’s invalid?

Some tools only check the record’s existence, not its format. MailTester validates the full syntax, including correct key wrapping and no line breaks.

How long does it take for a corrected DKIM TXT record to take effect?

DNS changes typically propagate within 1 to 24 hours, depending on the TTL value set. Full validation takes effect once receivers re-fetch the record.

Can I use DKIM without SPF or DMARC?

Yes, but DKIM only works at its best with SPF and DMARC. Without them, receivers may still reject or flag your emails, especially if other authentication signals are weak.

Is there a limit to how long a DKIM public key can be?

The full TXT value must be ≤255 characters per record. If the key exceeds this, it must be split correctly or stored using a single entry with no breaks.

Can MailTester fix my DNS record automatically?

No — MailTester detects issues in your DNS configuration, including malformed TXT records, but does not edit DNS. You must update the record manually in your DNS provider.

What happens if I don’t fix a malformed DKIM record?

Messages will fail DKIM validation, increasing the chance of spam filtering, rejection by receivers, and damage to sender reputation over time.

How often should I check my DKIM records for correctness?

Perform periodic checks when setting up new domains, after changing email providers, or after major infrastructure shifts — ideally every 3–6 months.

Can a typo in the selector name break DKIM?

Yes — if the selector name in the DKIM-Signature header does not match the TXT record name (e.g. 'default' vs 'mail'), validation will fail.