Sign in to your Google Admin console.

In the Admin console, go to Menu > Apps > Google Workspace > Gmail > End User Access

To apply the setting to everyone, leave the top organizational unit selected. Otherwise, select a child organizational unit.

Scroll to and click Warn for external recipients.

Check or uncheck the box to turn warnings on or off.

Click Save. If you configured a child organizational unit, you might be able to Inherit or Override a parent organizational unit's settings.