Mo. Rev. Stat. § 115.286
Absentee ballots under sections 115.275 to 115.304 received by the election authority in person or other authorized location designated by the election authority are deemed cast when received prior to election day. Absentee ballots received by the election authority through a common carrier such as the United States Postal Service are deemed cast when received prior to the time fixed by law for the closing of the polls on election day. Absentee ballots received by the election authority through a common carrier such as the United States Postal Service shall be received prior to the time fixed by law for the closing of polls on election day. The election authority shall hand mark or stamp each absentee ballot envelope as it is received, indicating the date and time the absentee ballot was received.
§ 115.286, RSMo