title: Firebase authentication Interception
body: <p>I am developing an App in flutter and firebase, which has a referral program included where by a user can refer another user and earn some point. The problem is I only want to give the point when the referred user verifies his/her email. 
  How can i intercept user verification process in firebase. 
  For example I want to give a referee a bonus when his/her downline verifies his email. </p>