A notice of cancellation of group life insurance coverage by an insurer due to nonpayment of premiums shall be in writing, shall be delivered to the policyholder or mailed to the policyholder at the last known address of the policyholder, shall state the effective date of the cancellation and shall be accompanied by a written explanation of the specific reasons for the cancellation. Proof of mailing of such cancellation notice shall be retained by the insurer for a period of not less than 1 year.