Mo. Rev. Stat. § 142.902

Any report required under this chapter which is transmitted through the United States mail shall be deemed filed and received on the date shown by the post office cancellation or if by certified mail the day shown on the certificate shall be deemed the postmarked date. If the report was mailed but not received, or when received the cancellation is illegible, erroneous or omitted, it shall be deemed filed and received on the date it was mailed if the sender establishes by competent evidence that the report was deposited in the United States mail on or before the due date.