By default, Gmail lets users automatically forward incoming mail to another address. As an administrator, you can turn off this option to prevent users from setting up automatic forwarding. If you turn off the option, users don't see the forwarding option in their Gmail settings and any existing forwarding rules or filters no longer forward messages.
Turn off automatic forwarding
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Sign in to your Google Admin console.
Sign in using your administrator account (does not end in @gmail.com).
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In the Admin console, go to Menu AppsGoogle WorkspaceGmailEnd User Access.
- To apply the setting to everyone, leave the top organizational unit selected. Otherwise, select a child organizational unit.
- Click Automatic forwardinguncheck the Allow users to automatically forward email to another address box.
- Click Save. If you configured a child organizational unit, you might be able to Inherit or Override a parent organizational unit's settings.