If there is a only a P.O BOX address can anyone serve papers there? and do law enforcements ever send you papers to a P.O BOX address? (Any kinds of letter, or even summons)
Some documents can be served by mail and others can't. If the PO address is the most current on record then they are likely to use it. Some documents require personal service, so obviously they can't do that through the mail.
If a document is sent by mail and not returned as undeliverable then it may be considered served.
MightyMoose also raises another issue - if you've agreed to receive service at a PO Box (which you can and may have done so by providing it as your address somewhere) then you've agreed to receive service of a summons and that ends any dispute!