215 ILCS 5/121-6
Any act of transacting an insurance business, as set forth in Section 121-3; by any unauthorized insurer constitutes an irrevocable appointment by that insurer, binding upon him, his executor or administrator, or successor in interest if a corporation, of the Secretary of State, to be the true and lawful attorney of such insurer upon whom may be served all lawful process in any action, suit, or proceeding in any court by the Director or by the State and upon whom may be served any notice, order, pleading or process in any proceeding before the Director which arises out of transacting an insurance business in this State by that insurer. Any act of transacting an insurance business in this State by any unauthorized insurer signifies its agreement that any lawful process in such a court action, suit, or proceeding and any such notice, order, pleading, or process in an administrative proceeding before the Director so served shall be of the same legal force and validity as personal service of process in this State upon that insurer.
215 ILCS 5/121-6