The joint tenancy has been severed upon the delivery of the notice of severance, regardless of whether Laura acknowledges it or not. Once the notice of severance is given, the joint tenancy is converted into a tenancy in common, and Laura cannot claim the entire property by the right of survivorship. Michael's estate is entitled to his share of the property. 

Key Point: Severance of a joint tenancy can be effected by giving notice, which does not require acknowledgment by the other party. Upon severance, the joint tenancy becomes a tenancy in common, and the right of survivorship no longer applies.