Mo. Rev. Stat. § 115.920: Ballot to be counted, when.

1. A valid military-overseas ballot cast in accordance with section 115.916 shall be counted if it is received before noon on the Friday after election day so that certification under section 115.508* may commence.

2. If, at the time of completing a military-overseas ballot and balloting materials, the voter has declared under penalty of perjury that the ballot was timely submitted, the ballot shall not be rejected on the basis that it has a late postmark, an unreadable postmark, or no postmark.