A malicious attack or misconfiguration can contribute to an unusually high rate of incoming email. To protect your users, Gmail monitors this incoming email rate and, when it reaches certain email receiving limits, automatically rejects messages to impacted accounts.

We want to provide G Suite admins increased visibility into this rate-limiting process. Going forward, if Gmail rejects 25 or more messages to a single recipient within one hour, we’ll send you a list of impacted users and and the number of their rejected messages. To receive this notification, you need to enable the rate-limiting email alert.


We hope this list, along with the Email Log Search feature, will help you identify why messages are being rate-limited and the appropriate actions to take.

Launch Details
Release track:
Launching to both Rapid Release and Scheduled Release

Editions:
Available to all G Suite editions

Rollout pace:
Full rollout (1–3 days for feature visibility)

Impact:
Admins only

Action:
Admin action suggested/FYI

More Information
Help Center: Email alerts


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Source: G Suite Updates Blog