§ 143.851 RSMo — Timely mailing

If any return, claim, statement, notice, petition, or other document required to be filed within a prescribed period or on or before a prescribed date under the authority of any provisions of sections 143.011 to 143.996 is, after such period or such date, delivered by United States mail to the director of revenue, or the officer or person therein with which or with whom such document is required to be filed, the date of the United States postmark stamped on the envelope shall be deemed to be the date of delivery. This section shall apply only if the postmark date falls within the prescribed period or on or before the prescribed date for the filing of such document, determined with regard to any extension granted for such filing, and only if such document was deposited in the mail postage prepaid, properly addressed to the office, officer, or person with which or with whom the document is required to be filed. If any document is sent by United States registered mail, such registration shall be prima facie evidence that such document was delivered to the person to which or to whom it is addressed. To the extent that the director of revenue shall prescribe by regulation, certified mail may be used in lieu of registered mail under this section. This section shall apply in the case of postmarks not made by the United States post office only if and to the extent provided by regulations of the director of revenue. When the last day prescribed under the authority of sections 143.011 to 143.996 , including any extension of time, for performing any act falls on a Saturday, a Sunday, or a legal holiday in this state, the performance of such act shall be considered timely if it is performed on the next succeeding day which is not a Saturday, Sunday, or legal holiday. The provisions of this section applicable to the timely filing of a document shall also apply to any payment required to be made under any provisions of sections 143.011 to 143.996 .