I've run into smelly drains several times at work.

You may think it's the hot water heater putting out that sulfur smell but often it's iron fixing bacteria growing in the drain. Hot water makes the smell worse than cold.

Pour in a cup of bleach, let it sit half an hour, flush it out with lots of water. If bleach won't get it sometimes you need bowl cleaner, which is an acid. Do not mix bleach and acid.