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SMTP codes

SMTP 4.7.26 Unauthenticated mail (Gmail, temporary)

Gmail: the message has no SPF or DKIM pass for the sender domain. Gmail requires authentication from all senders.

4.7.26
Class
4xx · Temporary failure
Retried?
Yes, the sending server retries
Defined in
RFC 7372 / provider

What to do

How it appears in a bounce

450 4.7.26 Unauthenticated mail (Gmail, temporary)

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

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