Andrea Anastasi's side had been installed as gold medal favourites by some after winning the World League, and appeared to be worthy of the tag as they took Italy apart in their opener on Sunday.
But Bulgaria, who beat Great Britain's in their first match, played them at their own game, hitting them off the court with a simple down-the-middle approach to win 22-25 27-29 25-13 23-25.
Tsvetan Sokolov led the way with 28 points. Poland were unable to deal with him at the net, while his four aces were an added bonus for the world number nines.
The former Beach Volleyball partnership of Zbigniew Bartman and Bartosz Kurek responded with a combined total of 41 points but, shorn of support in other areas, they were unable to keep their side in touch.
They have a chance to get back on track against Argentina on Thursday, while Bulgaria will be favourites to extend their unbeaten record against Australia.
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