Eggs produce a sulfurous smell when they are boiled for too long. Overcooking the egg coagulates the egg’s protein and creates an excess of hydrogen sulfide in the whites of the egg. The sulfide reacts with the iron in the yolk. This leads to an unpleasant odor and a green sulfur coat on the yolk.
A medium sized egg should ideally be cooked for for 11 to 12 minutes and will have less of a chance of producing a sulfurous smell and a green yolk. Also, plunging the boiled egg in ice water immediately after removing it from the pot will stop the internal cooking process, reduce the smell and keep the yolk golden yellow. Also a bonus bennefit of cooling the egg quickly makes the egg whites and yolk shrink slightly in size, making it easier to peel the shell.