Mo. Rev. Stat. § 474.170
The clerk of the court, after the will of a married person is admitted to probate, shall, within one month thereafter, mail by ordinary mail a written notice, directed to the testator's surviving spouse at his last known residence address, informing him that a written election must be filed by or on behalf of the surviving spouse in order to take against the will, within ten days after the expiration of the time limited for contesting the will of the decedent, unless the time is extended pursuant to law. Failure of the clerk to mail or of any surviving spouse to receive the notice herein required does not affect the time for making an election as prescribed by section 474.180. If the court is informed that a surviving spouse has been adjudicated a disabled or incapacitated person but has no guardian or conservator the notice need not be given but the court may appoint a guardian ad litem to make the election.
§ 474.170, RSMo