Washington Proof of Service by Mail is a legal method of delivering court documents and other legal notices to a party or parties in a legal action. The process involves sending a copy of the documents by certified mail, return receipt requested, to the party or parties in question. The return receipt serves as proof that the documents were delivered. This process is sometimes referred to as "Service by Mail" or "Proof of Service by Mail". There are two types of Washington Proof of Service by Mail: 1. Regular Service by Mail: This type of service is used when the party or parties can be found either at their home address or their office address. The documents are sent by certified mail, return receipt requested, to the party or parties' address. Once the documents have been delivered, the return receipt is filed with the court as proof that service has been made. 2. Substituted Service by Mail: This type of service is used when the party or parties cannot be found at either their home or office address. The documents are then sent by certified mail, return receipt requested, to the party or parties' last known address. If the documents cannot be delivered to the address specified, the documents are mailed to the party's attorney, if one is known, or to a family member, friend, or other person associated with the party. Once the documents have been delivered, the return receipt is filed with the court as proof that service has been made.