Wash. Rev. Code § 50.32.025
The appeal or petition from a determination, redetermination, order and notice of assessment, appeals decision, or commissioner's decision which is (1) transmitted through the United States mail, shall be deemed filed and received by the addressee on the date shown by the United States postal service cancellation mark stamped by the United States postal service employees upon the envelope or other appropriate wrapper containing it or, (2) mailed but not received by the addressee, or where received and the United States postal service cancellation mark is illegible, erroneous or omitted, shall be deemed filed and received on the date it was mailed, if the sender establishes by competent evidence that the appeal or petition was deposited in the United States mail on or before the date due for filing: PROVIDED, That in the case of a metered cancellation mark by the sender and a United States postal service cancellation mark on the same envelope or other wrapper, the latter shall control: PROVIDED, FURTHER, That in any of the above circumstances, the appeal or petition must be properly addressed and have sufficient postage affixed thereto.
RCW 50.32.025
Effective date-1975 1st ex.s. c 228: See note following RCW 50.04.355.