Illinois Summons affidavit Certificate of Mailing is an official document issued by the Illinois court system to verify that a Summons was properly mailed to the defendant in a civil lawsuit. It also serves as proof that the defendant was served with the Summons. This certificate is required to be filed with the court by the plaintiff or their attorney. There are two types of Illinois Summons affidavit Certificate of Mailing: one for service by mail and one for service by publication. The certificate of mailing by mail is used when the Summons is sent to the defendant via the US Postal Service. The certificate of mailing by publication is used when the Summons is published in a newspaper. Both certificates must be completed and signed by the person who served the Summons, and must include the date of service, the method of service, and the name and address of the defendant.