Why do you think a hand written notice is invalid? It is missing a signature. It is missing a submission date. It is missing an address. It is missing their last names. You know who it's from as it was submitted with their rent check. Do you not keep up with when your tenants lease's are comming up for renewal or ending? You should have been pro active and send tenant notice if they were renewing or not, and what move out and security deposit procedures are. Looks like you do not understand what you should be doing as a landlord.