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550 5.7.26

Sender not authenticated (SPF, DKIM, DMARC)

The receiving provider requires proof that your domain really sent the message, and your SPF, DKIM or DMARC checks failed or were missing. Large providers apply this rule strictly to bulk senders.

TypeSender or policy problem
Retry?Only after you fix your setup
The addressNot the problem: look at your sending

Common causes

  • No SPF record, or the sending service is not listed in it.
  • DKIM signing is not enabled for the domain you send from.
  • DMARC alignment fails because the From domain differs from the signing domain.
  • A forwarder or mailing tool rewrote the message and broke the signature.

How to fix it

  1. Publish one SPF record that lists every service that sends for you.
  2. Turn on DKIM in your sending service and publish its key in DNS.
  3. Add a DMARC record, starting with p=none and a report address.
  4. Send a test message and read the authentication results in the headers.

Frequently asked questions

Which record should I fix first?

SPF and DKIM first, then DMARC. DMARC only works when at least one of the other two passes and aligns with your From address.

Can a list clean-up solve it?

No. This is about your domain setup. Cleaning the list protects reputation, but authentication has to be fixed in DNS.

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