DKIM fail means the receiving mail server could not successfully verify the DKIM signature attached to an email. The problem can occur because the DKIM record is missing, the selector is incorrect, the public key does not match the private key, DNS has not propagated, or the message was modified after signing. A DKIM failure does not automatically mean an email will be rejected, but it can reduce authentication confidence and contribute to spam placement, especially when SPF or DMARC also fails.

DKIM is one part of modern email authentication. Gmail requires senders to authenticate their domains with SPF or DKIM, while bulk senders sending more than 5,000 messages per day to Gmail accounts must use SPF, DKIM, and DMARC. Authentication failures can result in spam placement or rejection depending on the circumstances.

What Is DKIM?

DKIM stands for DomainKeys Identified Mail. It is an email authentication method that uses cryptographic signatures to help receiving mail servers verify that an email was authorized by the domain shown in the DKIM signature and that signed portions of the message were not improperly changed during delivery.

When an email is sent, the sending system uses a private DKIM key to create a signature. The corresponding public key is published in the sender's DNS. The receiving server retrieves that public key and uses it to verify the signature.

A simplified DKIM process looks like this:

Email sender → DKIM signature → DNS public key → Receiving server → DKIM verification

If the signature can be verified, the receiving server can return a result such as:

dkim=pass

If verification fails, it may return:

dkim=fail

If there is no DKIM signature, the result may be:

dkim=none

Microsoft's authentication troubleshooting documentation identifies missing signatures, incorrect public keys, missing selectors, body modifications, and DNS lookup problems among the causes of DKIM-related failures.

What Does DKIM Fail Mean?

A DKIM fail means the recipient's mail system could not validate the DKIM signature associated with the message.

For example, a message header may contain:

dkim=fail

This does not necessarily mean the email is malicious. Legitimate messages can fail DKIM because of technical configuration problems or because an intermediary modifies the message after it has been signed.

A DKIM failure generally indicates that the receiving system could not establish a valid DKIM-authenticated identity for that message. The receiver then considers other authentication signals, including SPF, DMARC, sender reputation, message characteristics, and recipient engagement, when deciding what to do with the email.

DKIM verification failures do not inherently require a receiving server to reject a message. RFC 6376 states that a signature verification failure does not by itself force rejection.

What Causes DKIM to Fail?

There are several reasons why DKIM can fail. Some are caused by DNS configuration, while others occur because the message or signing configuration has changed.

1. The DKIM Record Is Missing

One of the most common causes is that the receiving server cannot find the public DKIM key in DNS.

A DKIM signature contains a selector, commonly represented by the s= tag. The receiving server combines the selector with the signing domain to determine where to look for the public key.

For example:

selector1._domainkey.example.com

If that DNS record does not exist, the receiving server cannot retrieve the public key required to verify the signature.

This can result in a DKIM failure or a missing-key condition.

2. The DKIM Selector Is Incorrect

The selector connects the DKIM signature to the corresponding public key in DNS.

For example, an email could contain:

d=example.com; s=selector1

The receiving server will typically look for the public key at:

selector1._domainkey.example.com

If the sending platform signs messages with selector2 while DNS only contains selector1, the receiver may not find the correct key.

This can happen when an organization changes email providers, adds a new sending platform, or manually updates DNS records.

3. The Public and Private Keys Do Not Match

DKIM depends on a matching key pair.

The sending server uses the private key to create the signature, while the receiving server uses the public key published in DNS to verify it.

If the DNS record contains a public key that does not correspond to the private key being used by the sender, verification fails.

This can happen after:

• Changing email providers

• Rotating DKIM keys incorrectly

• Replacing DNS records

• Copying the wrong TXT record

• Configuring multiple email platforms incorrectly

• Accidentally deleting or replacing an active DKIM record

Microsoft specifically identifies a mismatch between the public key in DNS and the private key used to sign the message as a cause of dkim=fail.

4. DNS Changes Have Not Propagated

DNS records do not necessarily become visible to every resolver immediately after they are changed.

If a DKIM record has recently been added or modified, some receiving servers may still obtain an older version of the record.

This can temporarily cause DKIM verification problems.

DNS propagation issues can be particularly confusing because the DKIM record may appear correct when checked from one location while another mail server still receives an outdated response.

5. The Email Was Modified After Signing

DKIM can also fail when signed content is changed after the sender creates the DKIM signature.

For example, an intermediary may modify the message body, headers, or formatting.

Microsoft lists mailing lists, transport rules, and intermediary services as possible sources of message-body modifications that can cause a DKIM body hash verification failure.

This is particularly relevant when email passes through multiple systems before reaching the recipient.

6. The DKIM DNS Record Is Invalid

A DKIM record can exist but still be unusable.

Potential problems include:

• Incorrect TXT syntax

• Missing quotation marks in some DNS interfaces

• Truncated key data

• Incorrect selector

• Extra characters

• Incorrect record name

• Improperly split long TXT values

• An outdated key

The important point is that a visible DKIM record does not automatically mean DKIM is configured correctly.

The receiving server must be able to retrieve a valid public key that corresponds to the signature.

7. DNS Lookup Times Out

The receiving mail server must query DNS to obtain the DKIM public key.

If the DNS lookup times out, the receiving system may be unable to complete DKIM verification.

Microsoft identifies oversized DKIM TXT records and slow authoritative DNS responses as possible causes of DKIM key lookup timeouts.

8. The Email Provider Is Not Signing the Message

Some businesses correctly configure a DKIM record but forget that the email platform also needs to sign outgoing messages.

Publishing a public key alone does not create a DKIM signature.

The sending service must use the corresponding private key to sign messages.

If it does not, the recipient may report:

dkim=none

rather than:

dkim=pass

This distinction matters because DKIM none and DKIM fail are not the same problem.

What Are Common DKIM Error Messages?

DKIM errors can appear in email headers, authentication reports, testing tools, or mail-server logs.

Here are some common results:

The exact wording can vary between receiving systems and email authentication tools.

Why Do Emails With DKIM Failures Go to Spam?

A DKIM failure does not automatically send every email to spam.

Instead, it can remove an important authentication signal from the message.

Receiving providers evaluate multiple factors when determining whether to accept, reject, quarantine, or place an email in spam. Authentication is one of those factors.

Gmail states that authenticated messages are less likely to be rejected or marked as spam and that messages that do not meet its authentication requirements may be marked as spam or rejected.

A DKIM failure becomes more important when other authentication or reputation signals are also weak.

For example:

DKIM fail + SPF fail + DMARC fail + poor sender reputation = higher delivery risk

By contrast:

DKIM fail + SPF pass + DMARC alignment through SPF + strong reputation = potentially different outcome

This is why fixing DKIM is important, but DKIM should not be treated as the only factor affecting inbox placement.

How Does DKIM Work With SPF and DMARC?

DKIM, SPF, and DMARC perform different jobs.

DKIM

DKIM adds a cryptographic signature to the message.

It helps the receiving system verify that the message can be authenticated using the public key published by the signing domain.

SPF

SPF identifies authorized sending servers for a domain.

The receiving server checks whether the sending IP is authorized by the domain's SPF policy.

DMARC

DMARC uses SPF and DKIM authentication results together with alignment requirements involving the visible From: domain.

This means a DKIM failure does not necessarily mean DMARC will fail.

If SPF passes and is properly aligned with the From domain, DMARC can still pass even when DKIM fails.

Conversely, if DKIM fails and SPF also fails or does not align, DMARC can fail.

For Gmail bulk senders, the domain in the From header must align with either the SPF domain or the DKIM domain for direct mail.

How to Fix a DKIM Fail?

Fixing DKIM requires identifying where verification breaks.

Step 1: Check the DKIM Authentication Result

Open the full email headers and locate the authentication results.

Look for something similar to:

Authentication-Results:

Then identify:

dkim=fail

Also check:

• d=

• s=

• header.d

• header.s

• Error or failure explanations

• SPF result

• DMARC result

The selector and signing domain are particularly useful because they tell you which DNS record the receiving server is trying to use.

Step 2: Identify the DKIM Selector

Look for the s= value in the DKIM signature.

For example:

s=selector1

The corresponding DNS record is generally located at:

selector1._domainkey.example.com

Check that this record exists for the correct domain.

Step 3: Check the DKIM Public Key

Confirm that the selector's DNS record contains the correct public key.

Do not simply check whether a TXT record exists. Confirm that the record belongs to the email platform currently signing your messages.

If your provider generated a new DKIM key, make sure the DNS record was updated accordingly.

Step 4: Check the Sending Platform

Log in to the email provider, CRM, marketing platform, transactional email service, or mail server responsible for sending the message.

Confirm that DKIM signing is enabled.

A DNS record without active DKIM signing will not produce a valid DKIM signature.

Step 5: Check for Multiple Email Providers

Many organizations send email through several services.

For example, a company might use:

• Google Workspace for employee email

• A CRM for sales messages

• A transactional email provider for notifications

• A marketing platform for newsletters

Each platform may require its own authentication configuration.

Adding DKIM for one provider does not automatically authenticate messages sent by another provider.

Step 6: Check DNS Propagation

If you recently changed the DKIM record, allow time for DNS changes to become available across recursive resolvers.

Also check the record from more than one DNS location when troubleshooting inconsistent results.

Step 7: Test the Actual Email

Do not rely exclusively on a DNS checker.

Send a real message from the affected sending platform to a test mailbox.

Then inspect the received message headers.

The goal is to confirm that the actual message shows:

dkim=pass

This verifies both sides of the process: the sender is signing the message and the recipient can validate the signature.

Why Choose EmailGuard for DKIM Fail Detection and Email Authentication Testing?

EmailGuard connects directly to your domain and instantly monitors SPF, DKIM, and DMARC records, flagging failures as soon as they occur rather than waiting for a manual check. If a record is missing or broken, its built-in lookup and generator tools let you fix it in the same interface. DMARC aggregate data further reveals exactly where and why DKIM failures happen over time. 

Beyond detection, EmailGuard verifies that fixes actually work, giving ongoing visibility instead of one-off snapshots. Its email tester scores messages across 162 deliverability data points, while DMARC insights track pass/fail history and sending sources per domain. With SOC 2, GDPR, and HIPAA compliance, plus free SPF/DKIM/DMARC tools for quick checks, it suits both hands-on teams and those wanting expert-managed setup and monitoring.

How long does it take to fix DKIM?

The technical change itself can be quick, but DNS changes may take time to become visible. After updating the record, allow DNS changes to propagate and then send new test messages.

What happens if the DKIM DNS record is missing?

The receiving server may be unable to retrieve the public key needed to verify the DKIM signature. This can result in a DKIM authentication failure or prevent DKIM verification altogether.

Does DKIM replace SPF?

No, DKIM and SPF are separate email authentication methods. Using both provides receiving mail systems with different signals for verifying legitimate email sources.

Is DKIM required for sending emails?

Email providers can have different authentication requirements, but DKIM is an important part of modern email authentication. Major mailbox providers increasingly expect senders to use authentication methods such as SPF, DKIM, and DMARC.

Can a DKIM failure cause a DMARC failure?

Yes, but not always. DMARC can pass through aligned SPF even when DKIM fails. If neither aligned DKIM nor aligned SPF passes, DMARC can fail.

How can I prevent DKIM failures in the future?

Keep your DKIM DNS records accurate, monitor authentication results, verify new email platforms before sending, maintain correct selectors and keys, and regularly test messages from your sending domains.