Simply receiving mail does not make you a resident. You must establish residency and that is being allowed access to the home, the understanding that you are leaving belongings behind with the ability to access them later, how long you have stayed, and maintaining things like utility bills. A lease is a contract that clearly establishes the guidelines between two willing parties. Absent that, the definition of residency is typically delineated in your state landlord-tenant laws.

Disclaimer: in the USA