Gmail unauthenticated domain rejection

Exact response pattern550 5.7.26 This mail is unauthenticated, which poses a security risk

PermanentGmailAuthentication

What it means

Gmail requires the message to authenticate and align with the visible From domain.

Likely causes

SPF and DKIM absent or failing.
DMARC alignment failure.

Recommended response

Fix SPF or DKIM with alignment.
Review Authentication-Results from a test message.

Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.

Related entries

454 4.7.0 — Temporary authentication failure

Generic · Authentication

530 5.7.0 — Authentication required

Generic · Authentication

534 5.7.9 — Application-specific password required

Generic · Authentication

535 5.7.8 — Authentication credentials invalid

Generic · Authentication

Browse all SMTP errors Decode a bounce