710 ILCS 30/1-15

(a) Unless otherwise agreed by the parties, any written communication is deemed to have been received if it is delivered to the addressee personally, or if it is delivered at his or her place of business, habitual residence, or mailing address. If none of these can be found after making a reasonable inquiry, a written communication is deemed to have been received if it is sent to the addressee's last known place of business, habitual residence, or mailing address by registered letter or any other means that provides a record of the attempt to deliver it. (b) Unless otherwise agreed by the parties, the communication is deemed to have been received on the day it is so delivered. (c) The provisions of this Section do not apply to communications in court proceedings.