735 ILCS 5/9-211: Service of demand or notice.

Any demand may be made or notice served by delivering a written or printed, or partly written and printed, copy thereof to the tenant, or by leaving the same with some person of the age of 13 years or upwards, residing on or in possession of the premises; or by sending a copy of the notice to the tenant by certified or registered mail, with a returned receipt from the addressee; and in case no one is in the actual possession of the premises, then by posting the same on the premises.