How to Fix DKIM Failure

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Understand DKIM failure consequences, common errors, and step-by-step troubleshooting to fix DKIM authentication issues and protect email deliverability.

by Maitham Al Lawati

Last Updated:
10 min read
How to Fix DKIM Failure
Table Of Contents−

DKIM failure for your emails can harm your domain reputation and impact your email deliverability. Fix all DKIM failures with this easy step-by-step tutorial.

Table Of Contents

Key Takeaways

  1. DKIM failures often arise from mismatches between the DKIM signature domain and the From header.
  2. Errors in DKIM record syntax, server communication, and DNS downtime can lead to DKIM authentication issues.
  3. Common causes of DKIM failure include improperly configured third-party vendors and modifications to the email body during transit.
  4. Utilizing reliable DKIM record generators and monitoring DMARC reports can significantly reduce DKIM failure instances.
  5. Implementing SPF and DMARC alongside DKIM helps enhance email security and ensures emails are properly authenticated.
  6. Failed DKIM authentication can result in emails being sent to spam folders, rejected, or bounced depending on recipient server policies.

DKIM failures can wreck inbox placement because they tell receiving servers your emails cannot be reliably authenticated. If you are seeing “DKIM authentication failed”, “dkim=fail”, or bounces like “550 DKIM validation failed,” it usually comes down to three causes: the DKIM key is missing or malformed in DNS, the message was modified after signing (forwarding, gateways, footers), or the DKIM signing domain does not align with the visible From domain under DMARC.

This guide explains what DKIM failure means, how to troubleshoot it step by step using email headers and DNS checks, and how to prevent repeat failures with proper vendor setup and DMARC reporting.

What Does DKIM Failure Mean?

DKIM (DomainKeys Identified Mail) failure happens when a receiving mail server can’t validate the DKIM signature on a message. In practical terms, the server checks the DKIM-Signature header, retrieves the sender’s public key from DNS using the selector (the s= tag), and verifies whether the signature matches what was signed (headers and body hash). If it is a match, DKIM passes for the message, or else DKIM fails.

DKIM failure refers to the failed status of your DKIM authentication check, due to a mismatch in the domains specified in the DKIM signature header and From header and inconsistencies among the key pair values.

What Happens When DKIM Fails?

When DKIM fails, the impact on email delivery depends on recipient filtering policies and whether DMARC is enforced on your domain.

In most cases, a DKIM failure increases spam risk rather than causing an immediate block. Receiving servers treat unsigned or unverifiable messages as lower-trust and may route them to the spam folder, especially if failures happen consistently over time.

Rejection typically occurs only when DKIM failure is combined with other authentication failures. If both DKIM and SPF fail, and your domain has a DMARC policy set to quarantine or reject, the message will fail DMARC and may be throttled, quarantined, or bounced with errors such as “550 DKIM validation failed.”

How DMARC changes the outcome

DMARC evaluates authentication differently than DKIM alone. To pass DMARC, an email needs only one aligned authentication method:

  • SPF passes and aligns → DMARC passes, even if DKIM fails
  • DKIM passes and aligns → DMARC passes, even if SPF fails
  • Both fail or don’t align → DMARC fails, action depends on policy

This means DKIM failure does not automatically break delivery, but repeated failures weaken sender reputation and remove an important layer of authentication protection.

Common Reasons for DKIM Failure

1. Error in DKIM record syntax

If you don’t use a reliable DKIM record generator and instead manually create the record for your domain, configuration errors are common. Syntax issues in DKIM DNS records, such as missing tags, broken quotes, truncated values, or extra spaces, can prevent receiving servers from validating the DKIM signature, resulting in DKIM authentication failure.

 2. DKIM Check for alignment failure

If you have DMARC set up for your domain in addition to DKIM, during DKIM check, the domain value in the d= field on the DKIM signature in the email header has to align with the domain found in the From address. It can either be a strict alignment, wherein the two domains have to be an exact match or a relaxed alignment that allows an organizational match to pass the check. 

A DKIM failure can occur if the DKIM signature header domain doesn’t match the domain found in the From header, which might be a typical case of domain spoofing or impersonation attack. 

3. You have not set up DKIM for your third-party email vendors

If you use several third-party email vendors to send emails on behalf of your organization, you need to get in touch with them for instructions on how to activate DKIM for your outbound emails. If you are using your own custom domains or subdomains registered on this third-party service to send emails to your customers, be sure to request your vendor to handle DKIM for you.

Ideally, if your third-party vendor is helping you outsource your emails, they would set your domain up by publishing a DKIM record on their DNS using a DKIM selector that is unique to you, without you having to intrude.

OR, 

You can generate a DKIM key pair and hand over the private key to your email vendor while publishing the public key on your own DNS. 

Misconfigurations in the same can lead to DKIM failure, so you must communicate openly with your service provider regarding your DKIM setup. 

Note: Some third-party exchange servers induce formatted footers in the message body. If these servers are intermediary servers in an email forwarding process, the conjoined footer can be a contributing factor to DKIM failure. 

4. Problems in server communication

In certain situations, the email might be sent from a server that has DKIM disabled on it. In such cases, DKIM will fail for that email, even if other servers in your infrastructure are correctly configured. It is important to ensure that communicating parties have DKIM properly activated. 

5. Modifications in message body by Mail Transfer Agents (MTAs)

Unlike SPF, DKIM doesn’t verify the sender’s IP address or return-path while verifying the authenticity of messages. Instead, it ensures that the message content has remained untampered in transit. Sometimes participating MTAs, and email forwarding agents may alter the message body during line wrapping or content formatting that may lead to DKIM fail. 

Formatting an email’s content is usually an automated process to ensure the message is easily comprehensible for each recipient. 

6. DNS outage / DNS downtime

This is a common reason for DKIM failures. DNS outage may occur due to a variety of reasons including denial of service attacks. Routine maintenance of your name server may also be the reason behind a DNS downtime. During this (usually short) period of time, recipient servers cannot perform DNS queries. 

As we know that DKIM exists in your DNS as a TXT/CNAME record, the client-server performs a lookup to query the sender’s DNS for the public key during authentication. During an outage, this is deemed not possible and hence may break DKIM. 

7. Using OpenDKIM

An open-source DKIM implementation known as OpenDKIM is commonly used by mailbox providers like Gmail, Outlook, Yahoo, etc. OpenDKIM connects with the server through port 8891 during verification. Sometimes, errors can be caused by enabling wrong permissions due to which your server is unable to bind to your socket. 

Check your directory to make sure you have enabled permissions correctly, or if at all you have a directory set up for your socket. 

Common DKIM Error Messages and Their Meaning

Understanding specific DKIM error messages helps you quickly identify and resolve authentication issues. Here are the most common DKIM failure messages and what they mean:

1. Authentication Result: dkim=neutral (bad format)

Auto-generated line breaks in your DKIM record can prompt the error message: dkim=neutral (bad format). When your email validator links together the broken-up resource records during verification, it produces a wrong value. A possible solution is to use 1024 bit DKIM keys (as opposed to 2048 bits) to fit within the 255-character DNS limit. 

2. Authentication Result: dkim=fail (bad signature)

A DKIM authentication failed result can be possible because of content modifications within the message body by a third party, due to which the DKIM signature header failed to match the email’s body. 

3. Authentication Result: dkim=fail (DKIM-signature body hash not verified)

The “DKIM-signature body hash not verified” or “DKIM signature body hash did not verify” are two alternative results returned by the receiving server for the same error that implies the DKIM body hash value (bh= tag) has somehow been altered in transit. Even if your DKIM key pair is set up correctly and you have a valid public key published on your DNS, minor modifications in the hash value, such as the insertion of spaces or special characters can make your body hash verification fail DKIM.

The bh= tag value may be altered due to the following reasons: 

  • Intermediary servers responsible for changing mail content 
  • Addition of email footers by your email service provider  

4. Authentication Result: dkim=fail (no key for signature)

This error may be the result of an invalid or missing public key in your DNS. It is imperative that you make sure both your public and private keys for DKIM match, and are set up correctly. Are you sure your DKIM DNS record is published and valid? Check it now using our free DKIM record checker.

How to Fix DKIM Failures and Prevent Them From Coming Back

It is not possible to address all the issues mentioned above simply because they cannot all be bypassed. However, we have assembled some useful tips you can deploy to minimize your chances of DKIM failing. 

  • Generate and publish the DKIM record correctly. Use a trusted DKIM record generator and copy-paste values to avoid syntax errors and truncated keys.
  • Validate your DKIM record in DNS. Check for missing selectors, formatting issues, and DNS propagation problems using a DKIM record checker.
  • Use SPF and DMARC alongside DKIM. DMARC can pass when either SPF or DKIM passes and aligns, which helps reduce false rejects when one method fails.
  • Enable DMARC reporting. Reports show which sending sources are failing DKIM and how often, so you can fix the specific stream instead of guessing.
  • Audit third-party senders. Confirm every vendor that sends using your domain is correctly configured to sign with DKIM and align with your From domain.
  • Monitor authentication performance continuously. Track DKIM failure trends over time using a DMARC reader dashboard to catch spikes before inbox placement drops.
  • Escalate when failures persist. If DKIM keeps failing after DNS and vendor checks, bring in expert support from PowerDMARC to review signing, routing, and intermediate mail handling.

Note that we have covered some common DKIM failure prompts and their probable causes while providing a possible solution around them. However, errors might pop up due to various underlying reasons that are specific to your domain and servers, that have not been covered in this article. 

You must build up your knowledge around authentication protocols, sufficiently, before you implement them at your organization or enforce your policies. DKIM fail, or failure in SPF, or DMARC validation can impact your email’s deliverability.  

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Impact of Email Forwarding on DKIM and SPF

Email forwarding often interferes with authentication because messages pass through one or more intermediary servers before reaching the final recipient.

Why SPF fails on forwarded emails

SPF validates whether the sending IP address is authorized to send mail for the domain in the envelope sender (Return-Path). When an email is auto-forwarded, the forwarding server becomes the apparent sending IP. If that server is not listed in the original sender’s SPF record, SPF will fail.

Note: SPF is evaluated against the forwarder’s sending IP, not the original sender’s server, which is why forwarded emails often fail SPF even when the original setup is correct.

What happens to DKIM during forwarding

DKIM can survive forwarding only if the message remains unchanged after it is signed. In practice, many forwarding services and security gateways modify emails by adding footers, disclaimers, or performing content rewrites. Even minor changes can invalidate the DKIM signature and cause DKIM authentication to fail.

  • Sign outbound mail with DKIM. DKIM improves the chances of forwarded emails authenticating when content is not modified in transit.
  • Use SRS (Sender Rewriting Scheme) on forwarding systems. SRS rewrites the envelope sender so SPF can validate correctly after forwarding.
  • Avoid adding forwarding servers to SPF records. This approach is difficult to maintain and may lead to SPF lookup limits.

Setting up DKIM along with SPF is a recommended practice, however, it is not mandatory.

  • If you don’t want to set up DKIM for your domains, yet you want to reduce SPF failure caused by forwarding, use SRS (Sender Rewriting Scheme) or a proper redirect method on the forwarding system. SRS rewrites the envelope sender (Return-Path) so SPF can validate correctly after forwarding.
  • Else, if you control the forwarding infrastructure and it uses fixed outbound servers, you can authorize those sending IP addresses in your SPF record. This approach is harder to maintain and may run into SPF lookup limits, so it is best used only in controlled environments.

Why DKIM Still Matters

DKIM is an email authentication method that proves two things to receiving servers:

  1. the message was sent by a server authorized to sign for the domain, and
  2. the signed parts of the message were not modified in transit.

That matters because inbox providers use authentication signals to decide whether to trust your mail. When DKIM fails repeatedly, you lose a key trust signal, which can lead to spam placement, throttling, or bounces, especially when DMARC is enforced.

If you’re fixing DKIM failures today, don’t stop at “it passes once.” Make sure every sending source (including third-party platforms) signs consistently, and monitor failures through DMARC reporting so the issue doesn’t return.

FAQs

1. What Is DKIM and why set it up?

DKIM (DomainKeys Identified Mail) is an email authentication method that adds a cryptographic signature to outgoing emails. Receiving servers verify the signature using the public key published in your domain’s DNS. If the signature verifies, it indicates the message was not modified after it was signed and that the email was sent through a legitimate DKIM signing setup for the domain.

DKIM is not a spam filter on its own, but it is a key trust signal used alongside SPF and DMARC to reduce spoofing and improve deliverability.

2. What senders are failing DKIM?

Failure is typical for senders who:

  • configure the protocol improperly
  • use 2048-bit keys for non-supported email providers
  • email contents were altered by a third-party intermediary during the message transfer

3. Can DMARC pass if DKIM doesn’t?

Yes, provided that SPF passes for the email. If you have configured DMARC and aligned emails against both SPF and DKIM mechanisms, you need to pass only one of the checks (either SPF or DKIM) to pass DMARC. However, if your DMARC alignment only relies on DKIM authentication, DMARC will fail and so will DKIM.

4. Why is my message blocked due to DKIM failure?

Messages may be blocked when DKIM fails if the recipient server has strict authentication policies or if your DMARC policy is set to “reject” and both DKIM and SPF fail alignment checks.

5. How do I check DKIM in email headers?

Look for the “DKIM-Signature” field in the email headers and check the “Authentication-Results” field for DKIM status. You can view email headers in most email clients by selecting “View Source” or “Show Original.”

6. Can DKIM fail if SPF passes?

Yes, DKIM and SPF are independent authentication methods. DKIM can fail due to signature issues while SPF passes based on IP authorization. This is why implementing both protocols provides better email security coverage.

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