65 ILCS 5/9-2-25
Where the residence of any defendant named in the commissioners' report is shown thereby to be outside of the State of Illinois, and the residence is stated therein, a copy of the specified notice shall be sent by mail to that party, at the address so given, at least 15 days prior to the return day of the summons. If the residence of any defendant is found to be unknown, as shown by the report and affidavit, a similar notice shall be sent to the person last paying taxes upon the premises, if his residence is stated in the report. Such service, publication, and notices shall be sufficient to give the court jurisdiction of all the parties whose land is to be taken or damaged, so as to determine all questions relating to the proceeding, and affecting the land described in the report.
65 ILCS 5/9-2-25