Mo. Rev. Stat. § 475.338: Service on foreign guardian or conservator.

1. Service of process may be made upon the foreign guardian, curator, committee or conservator by registered or certified mail, addressed to his last reasonably ascertainable address. Notice by ordinary first class mail is sufficient if registered or certified mail service to the addressee is unavailable. Service may be made upon a foreign guardian, curator, committee or conservator in the manner in which service could have been made under other laws of this state.

2. If service is made upon a foreign guardian, curator, committee or conservator as provided in subsection 1 of this section, he shall be allowed at least thirty days within which to appear or respond.