You won't need to change your email for the e-Transfer withdrawing option!
What we do is send you an email to the address associated with your Newton account via the Interac Request Withdrawal feature, and from there Interac takes over and you login to your bank portal as you normally would. We will be able to figure out that you sent the e-Transfer based on how Interac sends the information again.
Please send a request through the in-app help chat, or via https://newton.co/support if you have any questions.