IF the landlord had the correct address his letter sent late will not be effective as to the security deposit. HOWEVER, he does not lose his rights to claim damages to the apartment/house. You can file a small claims action, but he can file a counterclaim for the alleged damages and the Court can make that a setoff against the security deposit. You can hire an attorney as there is an attorney's fee provision if you are successful in getting the security deposit returned.
The answers given are limited to the facts as given and presumed by the answer itself. Without seeing actual written documentation or having a conference to more fully explore the issues, this short answer has only limited application. Make sure to seek legal counsel and provide all documentation to get assistance in making informed legal choices. Bstein@dcfsz.com, 305 377 1505