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What is OpenDKIM? A Simple Guide from Basics to How It Works

August 25, 2025
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What is DKIM?

When you send an email, how can the recipient be sure that it really came from you and wasn’t forged?
That’s exactly why DKIM (DomainKeys Identified Mail) was created.

DKIM works by attaching a digital signature to your email. The receiving server then uses the public key stored in your domain’s DNS to verify this signature.

  • If valid → the email is confirmed as genuine and unchanged during transmission.
  • If invalid → the email is likely to be flagged as spam or spoofed.

Simply put: DKIM is like an electronic stamp that confirms whether an email is authentic or not.

What is OpenDKIM?

OpenDKIM is an open-source software package used to implement DKIM on mail servers.
It is usually installed alongside Postfix or Sendmail, and serves two main functions:

  • Sign outgoing emails using the private key.
  • Verify incoming emails by checking the signature in the header.

With OpenDKIM, administrators can easily manage email authentication, reduce spam, and protect their domains from forgery.

Why should you use OpenDKIM?

  • Avoid the Spam folder: Emails with valid DKIM signatures are more trusted by Gmail, Outlook, and other providers.
  • Protect your brand: Prevent attackers from spoofing your domain for phishing campaigns.
  • Required for DMARC: If you want to apply DMARC policies (a stronger email anti-spoofing standard), DKIM is mandatory.
  • Free & easy to deploy: As open-source software, OpenDKIM is widely supported on most Linux mail servers.

4. How does OpenDKIM work?

Here’s the basic flow:

  • The administrator generates a pair of DKIM keys (private + public).
  • The private key stays on the mail server and is used by OpenDKIM to sign outgoing emails.
  • The public key is published in the domain’s DNS TXT record.
  • When an email is sent, OpenDKIM adds a DKIM-Signature line to the email header.
  • The receiving server checks the signature using the public key in DNS:
    • If it matches → the email is verified as authentic.
    • If not → the email is treated as suspicious or spam.

    Think of OpenDKIM as the keeper of the stamp for your organization — every outgoing email gets stamped to prove its authenticity.

    Conclusion

    In today’s world of rampant spam and phishing, implementing DKIM is no longer optional — it’s essential.
    And OpenDKIM is the best free, effective, and easy-to-use tool to help you:

    • Improve the trustworthiness of your emails.
    • Protect your business brand.
    • Ensure your emails are less likely to land in spam.

    In the next article, we can go deeper into installing OpenDKIM on Postfix or Sendmail, with detailed DNS configuration steps.

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