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SMTP Error 550 5.7.26: Sender is unauthenticated

Error✕ Not retryable · fix the payloadNetwork

TL;DR · 10-second read

Gmail rejected mail that passed neither SPF nor DKIM. Publish SPF for your sending IPs and sign with DKIM; bulk senders also need DMARC.

Identify: does your log match?

What you see · exact log / response

550-5.7.26 This mail has been blocked because the sender is
550-5.7.26 unauthenticated. Gmail requires all senders to authenticate with
550-5.7.26 either SPF or DKIM.
550 5.7.26 https://support.google.com/mail/answer/81126#authentication - gsmtp

Fix

Being written

Copy-paste fixes for 550 5.7.26 are in the enrichment queue

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Related errors

Last verified 2026-10-20 · Source: RFC 5321 reply codes + RFC 3463 enhanced status codes (IANA registry) · Official docs ↗ · ~5,400 searches/mo