Mo. Rev. Stat. § 136.357: Postal error cause for late return, no penalty assessed.

For purposes of the timely payment of any taxes imposed under the provisions of law and collected by the director of revenue, no additional tax or penalty shall be imposed on any taxpayer or other person or entity lawfully required to make payment of such tax to the director if the payment is delivered by United States mail and the postmaster for the jurisdiction where the payment was mailed verifies in writing the payment was mailed within the prescribed time period, including any extension granted, and was delayed because of an error of the United States postal service and not because of an error by the taxpayer.