When your users reply to an email message, the recipients may be outside your organization. You can set external recipient alerts to protect your users from unintentionally sharing information outside your organization (externally).
Gmail will detect if the external recipient in the email response is someone that the user is regularly communicating with or is not in user contacts. When you configure this setting, users receive notifications and the option to cancel.
Configuring external recipient response alerts:
From the home page, go to AppsGoogle Workspace Gmail> go lower and select End User Access
Tip: To see the Advanced Settings, scroll to the bottom of the Gmail page.
Select the top-level organization.
Go to Unintended external reply warning
Select this check box to enable or disable this setting.
It may take up to an hour for changes to be applied to user accounts. You can track previous changes in the Admin Console audit log.