550 #5.7.5 dkim unauthenticated mail is prohibited

A common error!

The error message "550 #5.7.5 DKIM unauthenticated mail is prohibited" indicates that the email server you're trying to send mail to is rejecting your email because it's not properly authenticated using DKIM (DomainKeys Identified Mail).

Here's what's happening:

  1. DKIM is a digital signature that verifies the authenticity of an email message. It's used to prevent spam and phishing attacks.
  2. When you send an email, your email server generates a DKIM signature using your domain's private key.
  3. The recipient's email server checks the DKIM signature to verify that the email came from your domain and hasn't been tampered with.
  4. If the DKIM signature is invalid or missing, the recipient's email server will reject the email.

To resolve this issue, you need to set up DKIM authentication on your email server. Here are the general steps:

  1. Generate a public and private key pair: You'll need to generate a public and private key pair using a tool like OpenSSL. The public key will be used to sign your emails, and the private key will be used to verify the signatures.
  2. Publish the public key: You'll need to publish the public key in the DNS records of your domain. This will allow the recipient's email server to look up the public key and verify the DKIM signature.
  3. Configure your email server: You'll need to configure your email server to use the private key to sign your emails. This will typically involve setting up a DKIM plugin or module on your email server software.
  4. Test your DKIM setup: Once you've set up DKIM, you'll need to test it to ensure that it's working correctly. You can use online tools like DKIM Analyzer or Mail Tester to verify that your DKIM signatures are valid.

If you're using a cloud-based email service like Gmail or Office 365, you may not need to set up DKIM authentication yourself. Instead, you can contact your email provider's support team to ask about their DKIM setup and configuration options.