When you are creating/editing the email/newsletter, in the Delivery Options section, you have the ability to change the sender and the reply-to address for outgoing newsletters and automated e-mail notifications.
In order to enable an e-mail address to act as sender or reply-to, you need to 1st set them up as a new user in the platform by going to the settings menu and click on manage users (admin only).
Then if the person is not a user, please invite them. Once they're setup as a user, you can click on the Request button under the Send From column.
This will e-mail them a request to gain access to send on-behalf of them.