If any lawful assessment is not paid within 30 days after written demand by the insurer or its agent, the directors may declare the policy suspended until the assessment is paid or may at their option sue for and collect the amount due on such assessment. Mailing such demand addressed to the insured at the insured's address last of record with the insurer, or delivering it to the insured in hand by an authorized agent or officer of the insurer, is conclusive proof that demand has been duly made.   [RR 2021, c. 1, Pt. B, §309 (COR).]