Washington Arbitrators Certificate of Mailing is a document issued by the Washington State Arbitration Council to certify that a particular document has been sent through the mail. It is used to prove that the sender has fulfilled their obligation to provide a document to the other party. There are two types of Washington Arbitrators Certificate of Mailing: Regular Certificate of Mailing and Registered Certificate of Mailing. The Regular Certificate of Mailing is used to prove that a document was sent via first-class mail, while the Registered Certificate of Mailing is used to prove that a document was sent via registered mail with a return receipt requested. Both types of Certificate of Mailing must be filled out completely to be accepted as evidence in an arbitration.