The summons and complaint must be served by a non-party (someone other than the landlord) and served into the hands of a tenant or other resident of the rental unit.
This is in contrast to the initial eviction notices. A 3-day notice to pay rent or vacate, 10-day notice to comply or vacate, or 20-day notice to terminate a month-to-month tenancy may be served by the landlord. These notices may be served by the landlord themselves, and on anyone, whether they reside in the rental or not, as long as copies are also mailed to the rental property. If no one answers the door, the notice may be posted, again as long as copies are also mailed to the rental property.
The next step in an eviction, the summons and complaint, may not be served by the landlord. The summons and complaint must be served on a resident. Also, the summons and complaint must be placed into the residents’ hands. Posting and mailing a summons and complaint requires a court order.
If the summons and complaint are served by posting and mailing, the landlord can evict the tenant and get the rental property back, but the landlord does not get a judgment for money owed in the eviction action (a.k.a. unlawful detainer action).
A bill was introduced in 2015 that would have allowed landlords to get a money judgment in eviction cases when the summons and complaint were served by posting and mailing, if the tenant responded to the complaint. However, the bill was never passed into law.
The landlord can get a money judgment if the summons and complaint are personally served (i.e., placed into the hands of a tenant or other resident).
If you have questions about the eviction process you should consult with an attorney.
 RCW 59.18.055. Negash v. Sawyer, 131 Wn.App. 822, 192 P.2d 824(2006).
 SB 5220.