A PO Box can only be used as a correspondence or mailing address in addition to the legally registered address of the entity or person applying for the trademark.
If you are inquiring about this option because of privacy concerns, know that certain IP offices only show the mailing address in the public records and hide the applicant's domicile address.
Also, once the trademark is registered, it is possible to change the owner's contact details by filing the Trademark Assignment Agreement. This procedure is free in some jurisdictions, while in others, it's paid. We always recommend inquiring with your legal representative if you wish to make this change.