How to Fix DKIM Errors — Full Troubleshooting Guide (2025)

Published on November 6, 2025 • by MailTested Team

A broken DKIM record can silently destroy your sender reputation. Even a single typo or missing character in your DNS record can cause your email signatures to fail — pushing your messages straight into spam. This guide explains how to identify, debug, and permanently fix DKIM failures with simple, real-world steps.

1️⃣ Understand What a DKIM Error Means

When you see “DKIM fail” in your mail headers or reports, it means the recipient’s mail server couldn’t verify your digital signature. This usually happens because your public key in DNS doesn’t match the private key your mail server used to sign the message. Some servers also reject messages outright if DKIM fails under strict DMARC alignment.

Run DKIM Check →

2️⃣ Common DKIM Failures and What Causes Them

a. Wrong or Incomplete DKIM DNS Record

A single missing quote or line break can corrupt your key. Your DKIM public key must be a continuous one-line string inside your DNS TXT record.

default._domainkey.mailtested.com  TXT  
"v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFA..."

b. Selector Mismatch

Each DKIM record uses a selector — a prefix that identifies the key pair. If your DNS record uses “default” but your mail server signs with “mail” or “smtp”, verification will fail.

c. Mail Server Not Signing Outgoing Mail

Even if your DNS is perfect, your MTA (Exim, Postfix, or Microsoft 365) must actually sign the messages. If DKIM signing is disabled, every message leaves unsigned — and automatically fails verification.

d. Key Length or Algorithm Problems

Keys shorter than 1024 bits are often rejected by major providers. Make sure your record uses k=rsa and a strong key (2048 bits or higher).

e. Corrupted Header Canonicalization

Some older systems or misconfigured relay servers alter line breaks or whitespace in the message body, invalidating the hash used for DKIM verification. Always check for intermediaries that modify messages in transit.

3️⃣ Step-by-Step: How to Fix DKIM Issues

Check your DKIM record with the MailTested DKIM Checker.
Verify selector — ensure the DNS and mail server match (for example, both use “default”).
Regenerate keys if your record was corrupted or truncated in DNS.
Update DNS with a single-line key (no spaces or breaks).
Enable DKIM signing in your mail server or sending platform (cPanel, Postfix, or Microsoft 365).
Test by sending an email to MailTested and verify the DKIM “pass” status.

Generate New DKIM Record →

4️⃣ Check for Related SPF and DMARC Failures

DKIM rarely fails in isolation. If your SPF or DMARC setup is also broken, mailbox providers will treat all your messages as untrusted. Run a full authentication test with MailTested.com to confirm all three (SPF, DKIM, and DMARC) align properly.

Run Full Deliverability Test →

5️⃣ Advanced Troubleshooting Tips

Use “dig” or nslookup to manually check your DKIM TXT record:
```
dig TXT default._domainkey.mailtested.com +short
```
Look for duplicate TXT entries under the same selector (only one should exist).
Review mail server logs — DKIM signing failures are often logged with reason codes.
If using a relay like SendGrid or Mailgun, ensure your domain’s DKIM keys are added in their console.

✅ Final Thoughts

Fixing DKIM isn’t guesswork — it’s about alignment and precision. Every valid signature increases your domain’s credibility with ISPs, improves your DMARC compliance, and strengthens your sender reputation. Once fixed, DKIM rarely breaks again — just don’t forget to rotate your keys annually.

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