Washington Proof of Mailing is a postmarking service offered by the United States Postal Service (USPS) in the state of Washington. It is a service that provides proof of the date and time of mailing for documents sent through the mail. The USPS accepts legal documents with Washington Proof of Mailing and will attach a postmark to the envelope or package to confirm the date of mailing. There are two types of Washington Proof of Mailing: Certified Mail and Registered Mail. Certified Mail allows for proof of mailing, delivery, and signature upon delivery. Registered Mail provides additional security and includes a tracking number that can be used to trace the package’s progress through the mail system.