When you use Universal Email you can decide what goes in the from. There is no way for it to automatically know who sent it, so you need to set the value dynamically to do this. When you are in the Universal Email interface you can use the lightning bolt icon to set the value dynamically, then point to the value that holds the from address.
Does the user enter their address when filling in the form? If so then you should use the binding for the form field that they enter their email address into.
