620 ILCS 5/63
Every order of the Department shall be served upon every person, municipality or other political subdivision to be affected thereby, except as otherwise provided by Section 60 of this Act, either by personal delivery of a certified copy thereof, or by mailing in the United States mail a certified copy thereof, in a sealed package with postage prepaid, to the person, municipality, or other political subdivision to be affected thereby; or, in the case of a corporation, municipality or other political subdivision, to any officer or agent thereof upon whom summons of a circuit court may be served in a civil action. Where any person, municipality or other political subdivision has entered an appearance by an attorney, or other representative of record, service may be made upon such person, municipality or other political subdivision by serving the attorney or other representative of record; and in any event, mailing in the United States mail, as hereinabove provided, shall constitute service, without additional proof of the receipt of the certified copies of the order.
620 ILCS 5/63