Top 10 Reasons Why DKIM Fails

Why DKIM and Email Authentication is important

Businesses run on email, and phishing attacks and email fraud work by forging it. A company needs a way to prove its messages are authentic and unmodified, and that is the job of email authentication, specifically DKIM (DomainKeys Identified Mail).

DKIM is an email authentication method that lets the recipient's mail server verify that a message came from the claimed sender and that its content was not modified in transit. Your mail server signs each outgoing message with a private key, and receivers check that signature against the public key you publish in DNS. A forged or tampered message fails the check, so no one else can send mail in your domain's name and have it verify.

The ten reasons below cover the most common ways DKIM breaks, each with a practical fix. Resolving them improves your email deliverability and protects your brand reputation, whatever volume you send.

Reason 1: Incorrect DKIM Configuration

A mistake made during setup is one of the most common causes of DKIM failure. Setting up DKIM means generating keys and configuring them correctly at both ends, and one wrong value breaks verification.

To fix this, start by verifying your DKIM settings. Double-check the configuration against the documentation from your email service provider (ESP) or IT team. If you find any discrepancies, update the settings. If you're uncertain what the correct configuration looks like, ask your ESP or IT team for help.

Reason 2: DNS Configuration Errors

DKIM relies on the Domain Name System (DNS) to publish the public key used for email verification. DNS configuration errors can lead to DKIM failures, as the receiving mail server may not be able to retrieve the DKIM public key.

To address this, check that your DNS records are correctly set up for DKIM. Compare the record values against the configuration provided by your ESP or IT team. Then confirm the DKIM record is actually published and reachable, so receiving mail servers can look it up.

Reason 3: Mismatched Key Pair

Using an incorrect or mismatched public-private key pair causes DKIM authentication failures. DKIM signs with the private key and verifies with the public one; if the two don't belong to the same pair, every signature fails.

To resolve this, generate a new key pair following the instructions from your ESP or IT team. Update your DKIM settings with the new keys and confirm they are correctly associated with your domain, so the recipient's mail server can verify your signatures again.

Reason 4: Email Forwarding and Third-Party Services

Email forwarding and third-party services can break DKIM authentication. When an email is forwarded and its content is modified along the way, the original DKIM signature no longer matches the message, so verification fails.

To overcome this, consider using SMTP forwarding instead of traditional email forwarding. SMTP forwarding preserves the original email headers and content, including the DKIM signature. Update your DKIM settings to account for SMTP forwarding.

Reason 5: Email Modifications during Transit

Email modifications made by intermediate servers or transit providers can impact DKIM signatures. If the email content is altered in any way, the DKIM signature verification will fail.

To minimize email modifications, consider end-to-end encryption methods such as S/MIME or PGP, which keep the email content intact throughout transit. Alternatively, enable DKIM signing by transit providers if they offer such a service. The transit provider then applies its own DKIM signature after making modifications, so the message still verifies on arrival.

Reason 6: Email Service Provider Limitations

Some email service providers may impose limitations on DKIM authentication, which can lead to failures. These limitations can be related to key size, signing algorithm, or other configuration options.

If you encounter such limitations, ask your current provider whether workarounds or configuration options exist to get past them. If none do, consider moving to an email service provider that supports the key sizes and signing algorithms you need.

Reason 7: Outdated or Incompatible Software

Using outdated or incompatible software versions for your email server can cause DKIM authentication issues. DKIM standards and best practices change over time, and old software falls behind them.

To fix this, update your email server software to the latest version compatible with the DKIM standards, so your implementation keeps up as the standards move.

Reason 8: Poor Reputation or Blacklisting

A poor sender reputation, or being blacklisted by email service providers, can undercut DKIM. These issues hurt email deliverability even when your other settings are right.

To mitigate this, actively monitor your sender reputation using reputation monitoring services or tools, and look up the domain on public blocklists so you catch a listing early. Keep your email hygiene in order: send relevant content to engaged recipients and handle unsubscribe requests promptly. If you find yourself blacklisted, resolve the issues that caused the listing and follow the delisting process provided by the respective blacklist authorities.

Reason 9: Inadequate Key Rotation

DKIM keys need regular rotation to stay secure and effective. Keys left in place too long put your authentication at risk and can lead to failures over time.

To address this, establish a key rotation schedule based on industry best practices. Rotate your DKIM keys at predetermined intervals, such as every six months or annually, and update the DKIM configuration with the new keys each time so authentication never lapses.

Reason 10: Lack of Monitoring and Maintenance

Without ongoing monitoring and maintenance, DKIM issues go undetected until messages start failing. A domain that passes today can drift as keys age, DNS records change, and standards move, so regular checks are what keep the implementation working.

Implement a monitoring system that regularly checks your DKIM authentication status and confirms it is passing. Establish maintenance procedures to address issues promptly and keep your DKIM implementation up to date with industry standards.

To see where a domain stands right now, run it through Palisade's DKIM Checker.

Use the result to pick which of the ten problems above to work on first. And when a failure has you stuck, your email service provider or IT team can confirm the configuration details on their side.