Section 43. The notice shall be served upon the insurer or an officer or agent thereof, or upon the insured, or upon one insured person if there is more than one, or upon any officer or agent of a corporation if the insured is a corporation, by delivering it to the person on whom it is to be served, or leaving it at his residence or place of business, or by sending it by registered mail addressed to the person on whom it is to be served, at his last known residence or place of business.