Mo. Rev. Stat. § 250.190

Any such city, town or village or sewer district operating a sewerage system or a combined waterworks and sewerage system under this chapter shall have power to supply water services or sewerage services or both such services to premises situated outside its corporate boundaries and for that purpose to extend and improve its sewerage system or its combined waterworks and sewerage system. Rates charged for sewerage services or water services to premises outside the corporate boundaries may exceed those charged for such services to premises within the corporate limits.