Iowa Admin. Code r. 721-25.16
25.16(5) Proof of mailing. Proof of mailing includes either a legible, nonmetered United States Postal Service postmark on the envelope, a certificate of service, a notarized affidavit, or a certification in substantially the following form:
I certify under penalty of perjury and pursuant to the laws of Iowa that, on (date of mailing), I mailed copies of (describe document) addressed to the (insert name of presiding officer) and to the names and addresses of the parties listed below by depositing the same in (a United States post office mailbox with correct postage properly affixed or state interoffice mail).
(Date)_______ (Signature)______________________