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SMTP codes

SMTP 5.7.26 DMARC / authentication failed

The message failed DMARC: neither SPF nor DKIM passed aligned with the From domain. Gmail: “Unauthenticated email is not accepted”.

5.7.26
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 7372 / provider

What to do

How it appears in a bounce

550 5.7.26 DMARC / authentication failed

Paste the full bounce into the decoder — it finds every code and recipient and says what to check.

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