Ability to emit EM wave

In my testbook, it says if the surface of a substance is dull then it is poor in emitting infrared and if it is shiny, then the opposite. But why? Shiny appearance makes light to bound off the substance, therefore making it hard to absorb it and emit it. And dull surfaces on the other hand should be easy to absorb light because it doesn't bound off it. So why saying dull surfaces are bad emitters of infrared? Or UV? Or visible light? Or EM waves? THanks!!!