[.:4:.]

My mum has an inbuilt setup, so do many offices i used to work in. In all those cases, replacing a bum microwave there was an exhaust vent that fitted to the rear that vented out the front of the trim surround via vents. There was 1 house though where the cupboard had a vent into the ceiling space. But if its supposed to be inbuilt, there should be some vent situation, otherwise its a potential fire hazzard.