Epsom salts are hydrated magnesium sulfate. My gut feeling is that they would produce a small amount of heat when dissolved in water, but I'm not positive of that.
Anhydrous magnesium sulfate does produce heat when added to water because water molecules bind to it (forming the hydrate). If the stuff you add is already hydrated, then you don't get the same release of energy.
There may be some heat released from the Mg and SO4 dissociating though.