where the deictic pronoun is important...

In general, when you don't know where the odor is coming from.
If not, Jane's got it right.

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Note added at 15 mins (2004-12-21 22:05:14 GMT)
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... and Nancy too for that matter.

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Note added at 17 mins (2004-12-21 22:06:55 GMT)
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... and Nancy too for that matter.

just an alternative word - refers to smelly decaying stuff. Not as good as rotten for more elaborate things :-) 'Something's putrid in the state of Denmark' - doesn't ring

"bad" in the specific sense of "rotten" (rancid etc.) - you would typically say "it smells bad" when sniffing something which has been in the fridge too long, to assess whether it's fit to eat.

"it smells off", "it smells a bit off" - again, used mainly with reference to food (i.e. the food has "gone off").

I'm suggesting these alternatives because they are in very common use, and these are the phrases I associate specifically with the word "pourri", but of course it's a question of context.
In the case of rubbish (garbage), then "stinks" is still the number one choice.

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Note added at 13 hrs 29 mins (2004-12-22 11:19:20 GMT)
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That\'s \"bad\" as in \"the food\'s gone bad\" (gone rotten).