Mont. Code Ann. § 33-1-612

Delivery, effectuation, or solicitation of any insurance contract, by mail or otherwise, within this state by an unauthorized insurer or the performance within this state of any other service or transaction connected with the insurance by or on behalf of the insurer must be considered to constitute an appointment by the insurer of the commissioner and the commissioner's successors in office as its attorney, upon whom may be served all lawful process issued within this state in any action or proceeding against the insurer arising out of any contract or transaction and must be considered to signify the insurer's agreement that any service of process has the same legal effect and validity as personal service of process upon it in this state.