Illinois Certificate of Mailing is a document issued by the United States Postal Service (USPS) that provides proof of mailing of a particular item. The USPS provides two types of Illinois Certificates of Mailing: a Standard Certificate of Mailing and a Registered Mail Certificate of Mailing. The Standard Certificate of Mailing is issued for any type of mail sent through the USPS and is used to document the date and time of mailing. The Registered Mail Certificate of Mailing is issued for Registered Mail items and provides proof of mailing and proof of delivery. Both certificates provide a tracking number for the item sent. The certificates are valid in all 50 states and are available at all USPS locations.