By statute, a divorce petition must be personally served. That is to say, someone must personally hand the documents to the responding party. It must be someone over the age of 18 who is not a party. So, it can be any adult that is not the petitioner. The person who does this deed (the “process server”) must then sign an affidavit (the “Affidavit of Service”) verifying that they have done so.
In the alternative to personal service, the responding party can acknowledge receiving the documents by signing an Admission of Service. Since the advent of email, this has become a common practice. The petition is PDF’d to the responding party, who is given the opportunity to sign an Admission of Service as a preferable alternative to being confronted by a process server, who may be someone they know or may be a complete stranger.
If the personal service happens outside the U.S., it is necessary (by statute) to have the process certified by a United States minister, charge d’affaires, commissioner, consul, commercial agent, or other consular or diplomatic officer.