The designation 'mailable' is determined by the geocode accuracy of the address, which is assigned through an integration with Google. More details about this designation are available in the response to this public issue, but in short, PO Boxes are not considered accurate enough to be 'mailable.'

For the time being, you can run a filter for "Address is mailable" OR "Primary Address... contains "PO" to include the PO Box addresses you'd like to reach in your mailable list.