Forward an inbox's received mail to another address

Set forward_to to an email address, or null to turn forwarding off. The account owner's own address activates immediately; any other address receives a confirmation email (sent from the inbox's verified domain and metered as one recipient) and forwarding stays pending until the link is confirmed. Forwarded copies are sent from the inbox address with the original sender as Reply-To, respect suppressions and plan limits, and are never re-forwarded.