Exchange: User Impersonation not working

Question

I'm trying to achieve impersonation for my Exchange subscription. I am the ADMIN for my tenant. I have followed these steps(http://www.codetwo.com/kb/how-to-set-impersonation-rights-manually/) to grant myself FULL CONTROL over other mail accounts for my tenant.

But when I try to access those through API, I get a 403 forbidden message.

URL

https://outlook.office365.com/api/v1.0/users/[email protected]

HEADERS

Authorization Bearer asdhasdjasd....

RESPONSE

{
    "error": {
        "code": "ErrorAccessDenied",
        "message": "Access is denied. Check credentials and try again."
    }
}

Following are the permissions that my APP asks for,

What am I doing wrong? I had this working about 6 months back, now when I'm trying it again, it doesn't work!