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.
Certain IP offices will show only the mailing address in the public records. However, in the US, for instance, adding the mailing address no longer hides the registered address.
Once the trademark is registered, it is possible to change the owner's contact details by filing the Trademark Assignment Agreement. This procedure is done free of charge in some jurisdictions, but in others, it's paid. We always recommend inquiring with your legal representative if you wish to make this change.