This problem is one that bothers my landlady a lot: All the apartments were built with space for a washer and a dryer, with the dryer next to an outside wall for venting the air.
But, I think it is her neighbor right next door (2 apts. per floor) who rearranged his apartment and decided to revent the dryer.
There is utility stack (right word?) that has floor-to-ceiling metal doors on each stairway landing. I didnt' stick my head in, because it's pretty crowded, but I guess they go from the bottom to the top of the building with no "floors" of their own. They carry the water drain, the power, probably the water, and maybe the gas and the phone.
They are 12 to 18" deep, probably 14 to 16" and the one she opened was about 30 or 36" wide, and there's another one just beside it. I think they have a cement (or concrete?) wall in back and maybe the sides, but if this could make a difference, I'm not sure and I can probably find out.
So this is where the n'bor vented his dryer. I think you can see the 4" hose in the closet, up near the ceiling of the landing, which would be half the distance from the floor to the ceiling of her apartment and his.
When she first said closet, I thought clothes, boxes, and storage. Not utilitiy closet. So I agreed that it was bad. But how bad is it to vent the dryer into this space? What reasons? There is some sort of condo association with rules, but of course, it if gets that far, one usually has to explain why something is really violating the rules.