If you don't want to show Joomla Email Configuration Address as sender Email Address please use this shortcode:
sender:firstname.lastname@example.org -> change to address you want to see as Sender/From in email clients.
yourAdminEmail@yourdomain.com -> your admin email address where the notification is supposed to be sent to.
You need to put this into your form settings' "Mail recipient(s)".
This is working for Mailback email, as well.
Warning: Use this at your own risk and with care. Make sure your sender address has the same domain part like your site is running on. For example, if your website domain would be www.mysite.com, then your sender email address should be something like email@example.com. If you use a different domain, then it may happen that emails that are sent out will be blocked due to suspected spam.