Can a PO box be used as a business address?
A P.O. Box is a mailing address by nature, and most states, as well as the IRS, that require listing a business address then it has to be a physical address.
Curiously, if you rent a UPS store address or an address of one of the mail forwarding companies, those would work, for no other reason but the fact that they look like physical addresses.
Still, those are not physical addresses in the sense that your business is not or cannot be physically located there. So I believe eventually those addresses might be singled out as not adequate to serve as a business address as well.