the closest thing you will get to a poisonous mammal is the slow loris. They have glands on the side of their elbow that secretes a toxin. This is used to cover their babies in to protect against predators, and and they mix it with their saliva to give them a venomous bite. So I guess that is poisonous.
venemous mammals are 2 Solenodon species;
platypus;
eurasian water shrew;
northern short tailed shrew;
shouthern short tailed shrew.