I think DJI makes this incredibly difficult and dos not tell people this needs to happen.
I bought a drone, actually traded for real estate photography and at the end the old owner reneged and would not release the drone nor give me payment in...
It shows you an Email?? What? Did the previous owner simply delete the app thinking that would unbind the Drone?...Tell them they gotta do it just as the above advice in post 2 says, or you wont be able to bind it to your account.