Wash. Rev. Code § 26.27.081
(1) Notice required for the exercise of jurisdiction when a person is outside this state may be given in a manner prescribed for service of process by the law of the state in which the service is made or given in a manner reasonably calculated to give actual notice, and may be made in any of the following ways:
(2) Proof of service outside this state may be made:
[ 2001 c 65 s 108.]