Through Gmail compliance and routing settings, Gmail administrators can now choose to notify internal senders when their messages get quarantined, depending on your compliance rule.
Below is an example of the email that users will receive when their emails get quarantined:
To enable the feature: In the Admin console where you setup and configure policies to quarantine messages, there is now a checkbox at the bottom to “Notify sender when mail is quarantined (onward delivery only)”. This feature is default off for existing rules, but when you create a new rule, the default is on.
Launching to both Rapid release and Scheduled release
Available to all G Suite editions
Gradual rollout (up to 15 days for feature visibility)
Reference by Google.com