A rotten egg smell in the water is one of the few household water complaints that can alarm a homeowner before they know the cause. Start by treating it as a question about where the smell lives, because that answer narrows the possibilities faster than any guess about the softener.
Check which tap carries it
Run the hot tap alone for a minute, then the cold tap alone. If the odor shows up only on the hot side, the water heater is the first suspect. Sulfur-producing bacteria can grow on the anode rod inside a tank heater, and warm water tends to bring the odor out. If both hot and cold carry it, the source is upstream of the heater, which puts the softener and the incoming supply both on the list.
Write down which fixtures you tested. Note whether the smell is strongest on the first draw in the morning or after the house has sat empty for a day. Those details will mean more to a technician than a general complaint about a bad smell.
Watch the timing around regeneration
Next, look at when the odor appears in relation to the softener's cycle. If it shows up in the hours right after a regeneration, the resin bed may be part of the problem, since bacteria can colonize a bed that stays wet between cycles. If the odor is present all the time with no link to regeneration, the source water is the more likely driver, and the softener is simply passing it through.
Ask the service provider whether the resin has ever been sanitized and how that is done. A proper cleaning uses a disinfectant suited to the system, and a good provider will explain what it involves and what the household should expect while it runs. Do not clean the resin tank yourself with household products unless the manufacturer's manual says to.
Test the supply before you blame the equipment
A water test answers questions that a hunch cannot. Ask for a sample of the raw water entering the house, ahead of the softener, and compare it with a sample taken after. If the raw supply tests positive for sulfide, the softener is not the origin, and resin work will not remove the problem at the source. If the raw sample is clean and the odor appears only after the softener, the bed or the drain line becomes the main suspect.
Label each sample with where and when it was drawn, and ask the lab or technician to record the sample point. A result without a sample location tells you very little.
When the smell is gas, not sulfur
Natural gas is given a rotten egg odor on purpose so that leaks can be noticed. If the smell is strongest near a gas appliance, at the gas meter, or in a room with no water fixture, stop testing water. Leave the house, keep the door open as you go, and call the gas utility or the emergency number from outside. Do not switch appliances on or off while you are unsure where the odor is coming from.
What to give the technician
When you call, give the provider the list you wrote: which tap carries the odor, whether it is hot, cold, or both, its timing relative to regeneration, how long the house had been empty, and whether the water heater has been serviced or replaced. That list lets a technician start with the right part of the system instead of working through the whole installation on the first visit.