-UserId1
  -Name: Yash
  -Email: dbs@gmail.com
-UserId2
  -Name: Tej
  -Email: tej@gmail.com
-UserId3
  -Name: Cal
  -Email: cal@gmail.com

DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
Query queryEmail = reference.child("user").orderByChild("Email").equalTo("cal@gmail.com");
queryEmail.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
               //Email already exist
            }else{
           //Email not exist
        }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });