No provision or condition of any insurance policy issued under the provisions of sections 380.201 to 380.591 and no right or defense of any company operating under the provisions of those sections shall be waived by such company or held to be waived by it, unless such provision, condition, right or defense is specifically waived by letter or other written or printed instrument purporting on its face that it is intended to be a waiver of such specified provision, condition, right or defense. The letter or other written or printed instrument, to be effective as a waiver, must bear the signature, not in facsimile, of an officer or other representative of the company authorized to execute such waivers. The letter or other written or printed instrument shall be the only admissible evidence of the waiver by the company of any such provision, condition, right or defense except that a specific admission under oath by a duly authorized officer or other duly authorized representative of the company that such letter or other written or printed instrument was intentionally delivered or deposited in the United States mail for the purpose of effecting such waiver may be received as evidence thereof.