Mo. Rev. Stat. § 429.100
Every person except the original contractor, who may wish to avail himself of the benefit of the provisions of sections 429.010 to 429.340, shall give ten days' notice before the filing of the lien, as herein required, to the owner, owners or agent, or either of them, that he holds a claim against such building or improvement, setting forth the amount and from whom the same is due. Such notice may be served by any officer authorized by law to serve process in civil actions, or by any person who would be a competent witness. When served by an officer, his official return endorsed thereon shall be proof thereof, and when served by any other person, the fact of such service shall be verified by affidavit of the person so serving.
§ 429.100, RSMo