Wash. Rev. Code § 51.04.082
Any notice or order required by this title to be mailed to any employer may be served in the manner prescribed by law for personal service of summons and complaint in the commencement of actions in the superior courts of the state, but if the notice or order is mailed, it shall be addressed to the address of the employer as shown by the records of the department, or, if no such address is shown, to such address as the department is able to ascertain by reasonable effort. If requested by the employer, any notice or order may be sent by secure electronic means except orders communicating the closure of a claim. Correspondence and notices sent electronically are considered received on the date sent by the department. Failure of the employer to receive such notice or order whether served or mailed shall not release the employer from any tax or any increases or penalties thereon.
RCW 51.04.082