One possible cause could be a lack of "high loop" between the sink and dishwasher. That is a pure guess, but worth a check.
The drain line connecting the sink drain to the dishwasher needs to run from the sink drain first up high to above the bottom of the sink then down to the dishwasher. If this "high loop" is not present, then the stuff put down the sink drain can go into the dishwasher. where it will rot and smell, particularly if you don't run your dishwasher very often.
As I said, that is just a stab in the dark. But I did have that problem in my villages house.
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