DomainKeys Identified Mail (DKIM) is an essential part of modern email authentication. It helps verify that your message hasn't been tampered with and that it truly comes from your domain.

However, DKIM can fail for several reasons — from DNS misconfigurations to message modifications in transit. Let's break down the most common DKIM problems, what causes them, and how to fix them.

1. DNS and Configuration Problems

Incorrect DNS Records

One of the most frequent DKIM issues involves typos or syntax errors in the public key record. Missing semicolons, extra spaces, or malformed key strings can prevent verification.

Fix: Double-check your DKIM TXT record for syntax accuracy. Use tools like the free MxToolbox DKIM Record Check to verify your public key.

Missing Public Key

If the DKIM public key isn't published in DNS, receiving servers can't verify the email signature.

Fix: Ensure your DKIM record exists and is publicly accessible. It should be located at:
selector._domainkey.yourdomain.com

Wrong Selector

The selector in your email header must match the one published in DNS. Even a small mismatch will cause verification failure.

Fix: Confirm that your mail server uses the correct selector and that the DNS record matches exactly.

DNS Propagation Delays

When you update your DKIM record, changes may take time to propagate globally. Sending emails before propagation completes can cause temporary failures.

Fix: Wait at least 24–48 hours after updates before testing or sending production email.

Incorrect Key Length

Keys that exceed DNS size limits can be truncated or broken across multiple records incorrectly.

Fix: Use a 1024-bit or 2048-bit key and ensure it fits within DNS character constraints.

2. Email Content and Transit Problems

Message Modification

If your message changes after signing, the DKIM hash won't match the body content — resulting in a "body hash did not verify" failure.

Common culprits include:

• Email forwarding systems that add disclaimers
• Anti-virus or spam gateways that rewrite content
• Inconsistent line breaks or whitespace changes

Fix: Use relaxed canonicalization and test with MxToolbox's free DKIM Record Lookup to identify where changes occur.

Canonicalization Mismatch

Strict "simple" canonicalization fails if even minor changes (like a new line break) occur during transit.

Fix: Switch to relaxed/relaxed canonicalization for better resilience.

Expired Keys

Expired DKIM keys will immediately cause signature failures.

Fix: Rotate and renew your DKIM keys regularly to maintain valid authentication.

3. Server and Implementation Issues

Mail Server Misconfiguration

Improper DKIM setup on your sending or receiving server can break validation.

Fix: Confirm your mail service is correctly signing messages and that the DKIM header appears in the raw email.

Missing or Altered Headers

Certain headers (like From, To, Subject) must be included in the signature.

Fix: Ensure these headers are part of your signing configuration.

Network or Communication Failures

DNS timeouts, blocked ports, or poor server connectivity can also trigger DKIM validation errors.

Fix: Check DNS resolution speed and firewall settings to ensure smooth server communication.

DKIM is crucial for protecting your domain reputation and ensuring email deliverability, but it can get complicated fast.

With MxToolbox Delivery Center, you can:

• Monitor DKIM, SPF, and DMARC configurations in one place.
• Get alerts for record errors or key expirations.
• Analyze failed signatures and deliverability trends.

Small DKIM issues can cause big deliverability problems. By auditing your DNS records, keeping keys updated, and monitoring for configuration drift, you'll ensure your messages always pass authentication — and make it safely to the inbox.

Check your DKIM record now!