Service of the notice and order shall be made upon all persons entitled thereto under HBC 8.12.080, personally or by mailing a copy of such notice and order by registered or certified mail, postage prepaid, return receipt requested, to the person’s address as it appears on the last assessment roll of the borough or to such proper address as is known, or reasonably ascertainable by, the abatement official. If no address of such person is available, a copy of the notice and order shall be addressed to such person at the address of the property involved in these proceedings. A failure of any such person to receive such notice shall not affect the validity of any proceedings taken under this section. Service by certified or registered mail in the manner herein provided shall be effective on the date of mailing.