The notice herein mentioned shall be in writing, directing the owner or occupant of premises adjoining such nuisance to abate the same by removing such obstacle impeding drainage, and shall be given by mailing to the owner or occupant of such adjoining premises at his or their last known post office address, or to both, if their names and post office addresses can be ascertained with reasonable diligence, by registered or certified mail or by delivery of such notice to such owner or occupant personally by any officer, employee or agent of the City or by posting such notice at some conspicuous place upon such premises if the name or mailing address of the owner or occupant of said premises cannot be ascertained with reasonable diligence. (1988 Code § 14-303)