MailFixIt

SMTP DKIM acceptance error

SMTP 550 5.7.21: No acceptable DKIM signature found

At least one DKIM signature passed, but the receiver accepted none of them for its policy. This differs from 5.7.20, where no DKIM signature passed.

Problem description

The registered response text is:

550 5.7.21 No acceptable DKIM signature found
Is this temporary or permanent?

SMTP 550 is permanent for this message. Retrying the same signed content will not change the receiver's decision.

Find the receiver’s acceptance rule

  1. Save the complete rejection and original message source.
  2. List every DKIM signature and its d=, s=, i=, and a= values.
  3. Identify which signatures passed in Authentication-Results or receiver diagnostics.
  4. Read the provider’s text after 5.7.21 for the rejected identity or policy condition.
  5. Configure the legitimate sender to meet that specific condition, then sign and send a new message.

Do not rotate a DKIM key merely because this code appears. A missing or invalid key normally prevents the signature from passing; 5.7.21 says a signature already passed but was not acceptable.

Frequently asked questions

Does SMTP 5.7.21 mean the DKIM signature failed?

No. The registered meaning says one or more DKIM signatures passed, but the receiver considered none acceptable.

How is SMTP 5.7.22 related?

5.7.22 is the specific case where passing signatures are unacceptable because none match the author address in the From header.

Related email diagnostics

SMTP 550 5.7.20: No passing DKIM signature foundUnderstand SMTP 5.7.20, inspect the DKIM selector and public key, and fix a missing or failing DKIM signature without guessing DNS values. SMTP 550 5.7.22: No valid author-matched DKIM signature foundFix SMTP 550 5.7.22 when DKIM passes, but no valid signature identifies the author in the From header. DKIM public key record is invalid or revokedDiagnose an invalid or revoked DKIM public key record. Check the exact DKIM selector in public DNS before replacing a sender-provided key.

Primary sources: IANA SMTP Enhanced Status Codes, RFC 6376: DomainKeys Identified Mail Signatures, RFC 7372: Email Authentication Status Codes.