Its an illegal catch ...
Michael Murray: "Re the Boult "catch" to dismiss Pollard, Law 32.3 (e) states that a catch shall be considered to be fair if "...after being struck by the bat, the first contact with the ball is by a fielder, not touching or grounded beyond the boundary, who has some part of his person grounded within the boundary or WHOSE FINAL CONTACT WITH THE GROUND BEFORE TOUCHING THE BALL WAS ENTIRELY WITHIN THE BOUNDARY". So should actually have been awarded 6 runs."
DWolfman throws in the law book: "Law 19:4 The ball may be caught, subject to the provisions of Law 32, or fielded after it has crossed the boundary, provided that i) the first contact with the ball is by a fielder either with some part of his person grounded within the boundary, or whose final contact with the ground BEFORE touching the ball was within the boundary. Unless I am reading that wrong then that Boult catch should have not have stood and that law was amended precisely to clarify this. "