A Powerform has both a Owner and a Sender, typically when the Template is first created the creator is the Owner, when you then create the Powerform the Owner remains the creator and now the Sender is also the same as the Owner by default. Powerforms can be changed so there is a different Sender but that Sender still needs to be a DocuSign User on the Account it cannot be a random email address. To change the Sender of the Powerform, locate the Template/Powerform and select it, then scroll to the bottom where is shows Associated Powerforms and select the Actions dropdown to the right and select "Change Sender", change and Save.