Sending outgoing bigpond.com emails to a back-up account

I can automatically forward incoming emails easily from my new bigpond email account to a back up account in the mail settings. I cannot find a way to similarly automatically forward the emails I send from this account to the back up account. The technical support team at Telstra told me that they were not allowed to tell me how to do this (go figure), but said it could be done. Does anyone know how to do this please?