Sulfur smells in your sink could be coming from the water or the drain, and either way, they aren't pleasant. The smell comes from hydrogen sulfide gas, which is produced by bacteria in the ground, in sewage and in contaminated water and pipes. It's often known simply as sewer gas. Once you determine whether it's in the water or the drain and whether it's confined to only that sink, you can figure out what to do about it.
Conduct a Few Tests
The first step in diagnosing the problem is to determine the smell's origin.
Is It in the Water or the Drain?
Fill a glass from the cold water tap, take it outside and smell it. Repeat this test with the hot water. If you smell the odor in both samples, there's a good chance the contamination is in the water supply. If you smell it only in the hot water, suspect the hot water heater, and if you don't detect the odor in either sample, it's probably coming from the drain.
Is It Just This Fixture?
If you detect the odor in the water, check the water in other fixtures in the house. If only the water in that sinks smells, the problem could a localized plumbing condition -- perhaps a "dead leg," which is a short length pipe that has been capped and is no longer used. Bacteria can grow in such pipes. If it's coming from other faucets, suspect contamination in the water supply.
If the odor is coming from the drain -- and it's only that drain -- it's reasonable to suspect contaminated waste pipes. The problem could also be related to blocked vents, especially if the smell gets stronger after you flush a toilet or empty the washing machine. Venting issues typically affect more than one fixture.
What to Do
Now that you've determined from where the sulfur odor is coming, you can plan a strategy to eliminate it.
Hot Water -- More Than One Fixture Affected
Your water heater is probably contaminated. The contamination often comes as magnesium in the anode rod reacts with the bacteria that cause the odor. Replace the rod with an aluminum one and disinfect the water in the tank with hydrogen peroxide, which is safer than chlorine bleach, which will also do the job. You can also remove the anode rod altogether, but this shortens the life of the water heater.
Cold Water -- More than One Fixture Affected
If you have a well, the pressure tank could be contaminated, and if so, disinfect it by adding household bleach in the proportion of 1 gallon for every 1,000 gallons of water. You may also have to disinfect the well. Contaminated water softeners are often the cause of odors -- replace the filter. If the problem isn't serious, running the water before you use it may flush the bacteria in the water and dissipate the odor.
Hot or Cold Water -- Just the Sink Is Affected
Your plumbing may have been renovated in the past, and a pipe that was no longer needed was capped. Because the water doesn't flow in that pipe, anaerobic bacteria that produce hydrogen sulfide gas can flourish there. Cut this "dead leg" out of your plumbing.
Sink Drain Accompanied by Drainage Problems
A vent blockage is probably interfering with water flow. When this happens, drainage from the toilet or another large water-using fixture creates pressure that either pushes sewer gases through the sink P-trap or sucks water out of the trap, allowing the gases to escape. You'll probably also notice gurgling sounds and slow draining. The vent blockage may be caused by an obstruction in the waste line, so plunging the nearest toilet may help. You should also go on the roof, inspect the vent stack opening and remove debris. If the problem occurs in winter, the vent may be iced over. Go in the attic an heat the vent pipe near the roof with a hair dryer, and consider replacing the vent stack with wider-diameter pipe in the spring.
Contaminated Sink Drain
Disinfect the drain to kill odor-causing bacteria in the waste pipe and P-trap. You can do this by pouring 1/2 cup of bleach into the drain. To disinfect the overflow, fill the sink with water, pour in 1/2 cup of bleach and keep filling the sink until water starts to overflow. Let about a cup of water flow, then wait for five minutes before draining the sink. If you're reluctant to pour bleach down your drain, pour in 1/2 cup of baking soda followed by a cup of vinegar. Let it fizz for 10 minutes, then rinse with 4 cups of boiling water. You may also want to disassemble the P-trap and give it a good cleaning with bleach or baking soda and vinegar.