In the future, I would recommend the following: When washing the mussels before cooking, if any are slightly open try gently closing them. If they react by snapping closed themselves or they stay shut, it means the mussels are alive and well. If instead they react by immediately popping back open, throw them out (they are dead). I've found that the majority of the dead mussels are slightly open before cooking; they rarely are completely, tightly closed. Using this method, when you actually cook the mussels, you have a reasonably good idea that they are all alive and therefore safe to eat, regardless of whether or not they open all the way during cooking. After cooking there will likely be none that are closed, but if there are, there will likely be few and you can discard them if you like.