Have you ever wondered why your sink can smell so badly from time to time? Your bathroom sink that gets the most usage?
There is an oft forgotten feature in your sink that needs regular cleaning like your toilet needs regular cleaning.
That smell is in the sink overflow canal. It has to be cleaned out with a cleaner and flushed regularly, just like a toilet does. There is mold and bacteria building up between the overflow hole and the pop up drain canal. Remember, that it is designed to take away excess water if the faucet is left on. Since most people turn off the faucet, it does not get used much, if at all. Moisture from sink usage finds its way into the overflow canal/tube during normal usage. With the lack of sunlight and fresh airflow, mold builds up and decomposes, creating the smell.
Cleaning on a regular basis will prevent the smell from returning. Try an anti-mildewcide sprayed down the tube and into the pop up drain area. Let it go to work, then connect a hose from the faucet to the overflow opening and power spray to ensure all sides are flushed. Do this regularly when the toilet gets cleaned and you should not experience it again.