I have a problem with plumbing that is 4 years old - I had mold & mildew smell in my bathrooom. I finally had the plumber in that did the bathroom. The mold was in the overflow drain. I cleaned it with bleach and the odor went away, but I now smell it again. The plumber said that by using the bleach it should have cleared up. He says there is no way it could be in the catch drain. I think it is. Could there be something caught in the drain that could be at the bottom of the overflow drain that continues to grow mold?

Anything moist and dark with a "food" source can grow mold. Your drains have it all since hair and body oils and dirt are food for mold. They eat almost anything organic. Remove the stopper from the sink and check for debris. Bleach again with 1:3 ratio of fresh bleach to water and allow to soak for 15 minutes before flushing with plenty of clean water. Pour some of that bleach into the overflow hole, it won't fill so soaking won't be happening but it's a try at least.