California Proof Of Service By Mail is a process used in California to serve legal documents by mail. It is a form of alternative service which is used when the defendant or respondent cannot be served in person. There are two main types of California Proof Of Service By Mail: Substituted Service and Service by Mail. Substituted Service is when documents are served on a person who is not the defendant or respondent, but who is related to the defendant or respondent—usually a family member or an adult living in the same residence. The documents must be given to the third party with a cover letter explaining the nature of the documents. The third party must sign an acknowledgement of receipt of the documents. Service by Mail is when documents are sent to the defendant or respondent by mail. The documents must be sent by certified mail and must include a cover letter explaining the nature of the documents. The defendant or respondent must sign an acknowledgement of receipt of the documents. California Proof Of Service By Mail is a process used to serve legal documents in California when the defendant or respondent cannot be served in person. There are two main types of California Proof Of Service By Mail: Substituted Service and Service by Mail.