I describes the total energy potential intrinsic to any body of matter, which is equal to the matter's mass times the speed of light squared. So while any given amount of energy does indeed have a sum potential mass, the intrinsic mass of something like electromagnetic radiation, for example, is equal to zero. In this way, photons do have mass potential at a constant "exchange rate" as defined by Einsteins equation, but once the mass becomes manifest as a property of the photon, well, it's not a photon anymore. It becomes matter.