Gmail temporary rate limit

Exact response pattern421 4.7.0 Temporary System Problem. Try again later

TemporaryGmailReputation

What it means

Gmail is temporarily deferring the message. The exact diagnostic and URL parameters determine whether the cause is rate, reputation, authentication or content.

Likely causes

New or unstable sender reputation.
Rapid volume increase.
Poor complaint or bounce signals.
Authentication problems.

Recommended response

Reduce rate and concurrency.
Check Postmaster Tools trends.
Confirm SPF, DKIM and DMARC.
Resume with a controlled ramp.

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

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