Under what condition can a person be served by mail or national courier?

A person can be served by mail or national courier if the serving party is aware of the recipient's address. This condition is essential because it establishes that the serving party has the necessary information to ensure that the documents are directed to the correct location. Serving documents through mail or a national courier relies heavily on the accuracy of the recipient's address to achieve effective service, enabling the legal process to move forward.

While other options might suggest specific scenarios under which service could be carried out, they do not meet the general prerequisite of having knowledge of the recipient's address. For example, serving a person outside the United States often involves different procedures or requirements that may not include service via mail or national courier, while the presence of a court-appointed guardian or the age of the recipient introduces additional legal factors that do not directly pertain to the method of service by mail. Thus, knowing the address remains the fundamental criterion for mailing or courier service of legal documents.