Ill. Admin. Code tit. 89, § 508.80 – Service

a) Notices under Section 508.50(a) shall be served either personally or by certified mail upon all parties or their agents appointed to receive service of process unless the applicable statute or rule requires a different form of service, in which case service shall conform to the statute or rule.
b) Service to the last official address of a party, or agent provided to the Department by a party, shall be considered in compliance with this Section. Notices sent by certified mail to that address and that have been returned to the Department as unclaimed or refused by the addressee shall be considered served. For purposes of this Section, the "last official address" shall be: the address listed on the most recent application submitted to the Department, unless the Department has been subsequently notified in writing of a change of address.
c) Service of pleadings or motions under this Part, unless otherwise provided for, shall be made by delivering in person or by depositing with the United States Postal Service, properly addressed with postage prepaid, one copy to each party to the proceedings. When any party has appeared by attorney, service upon the attorney shall be deemed service upon such party. All pleadings or motions under this Part shall also be served upon the administrative law judge.
d) Proof of service under subsection (b) of this Section shall be by certificate of attorney, affidavit or acknowledgment.