92 Ill. Adm. Code 454.140
Service
Section 454
Section 454.140 Service
a) Each order, notice, or other document required to be served
under these Rules shall be served personally or by first class, registered or
certified mail, except as otherwise provided.
b) Service upon a person's duly authorized representative
constitutes service upon that person.
c) Service by registered or certified mail is complete upon
mailing. An official United States Postal Service receipt for registered or
certified mail constitutes prima facie evidence of service.
d) One copy of each order, notice or other document must be
served on each party. Service on any party shall be by the same means employed
in serving all other parties.