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SMTP error X.7.21

No acceptable DKIM signature found

Updated Jul 1, 2026

The short answer

No acceptable DKIM signature found Common cause: DKIM passes technically but the receiver does not accept the signature

Quick fix

Check signing domain, selector, alignment, and receiver policy

This code is written X.7.21 because the reason is the same whether the failure is temporary (a 4 first digit) or permanent (a 5). Read the first digit of your own bounce to know which applies.

How to fix it

Check signing domain, selector, alignment, and receiver policy

Related codes

X.7.0 Other or undefined security status X.7.1 Delivery not authorized X.7.2 Mailing list expansion prohibited X.7.3 Security conversion required but impossible X.7.4 Security features not supported X.7.5 Cryptographic failure
All SMTP error codes
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