Although several conditions may cause sink drains to smell bad, one of the most common is bacterial growth fueled by food particles that stick to the sides of the drain. SF Gate recommends a weekly cleaning with baking soda and vinegar followed by a rinse with boiling water. Food clinging to the stopper is another source of odor. Cleaning the stopper stops the odor.
If the sink has a removable stopper mechanism, a bottle brush is useful for cleaning the upright portion of the pipe. The brush loosens the biological slime that adheres to the sides of the pipe and allows it to wash down the line. If the stopper assembly is not removable and the baking-soda, vinegar solution does not remove the smell, a one-to-10 solution of bleach and water flushed down the drain kills the odor-causing bacteria.
If the odor is coming from a sink in a seldom-used, remote bathroom, the problem is often evaporation of water from the trap. The trap holds a small amount of water, which keeps sewer gases from rising through the drain. If the trap water evaporates, these gases may vent through the drain and cause odors. Running water in the sink once a week replenishes the trap and prevents the odor.