Service of the notice and order shall be made upon all persons entitled thereto either personally or by mailing a copy of such notice and order by registered or certified mail, postage prepaid return receipt requested, to each such person at his address as it appears on the last equalized assessment roll of the city or borough or as known to the abatement official. If no address of any such person so appears or is known to the abatement official, any copies of notice or order shall be so mailed addressed to such person, at the address of the property involved in these proceedings. The failure of 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.