Some musts/yeast combos produce a lot of So2. So you get a rotten egg/sulfery smell. This is a sign of stressed yeast often. Did you use yeast nutrient & energizer like DAP, nutriferm advanced, fermaid-k......? Also degassing or oxygenating up to the 1/3 sugar break helps a lot. With time and racking it should dissipate.
A painting says a thousand words. But a painting while on good mead just looks funny!