Private companies may uniquely identify, by number, individuals or offices within their company for the purpose of simplifying their internal mail distribution. Print this number, called a MailStop Code (MSC), above the Recipient Line or the Attention Line. The words POST OFFICE BOX or PO BOX and the private mailbox number cannot be used on the Delivery Address Line. Only the Postal Service is entitled to provide delivery to a PO Box.
Private companies offering mailbox rental services to individuals and businesses are considered commercial mail receiving agencies (CMRA). Addresses on mail received at a CMRA must adhere to specific requirements in the use of their private mailbox number (PMB). Mail sent to an addressee at a CMRA must include either the PMB identifier or the #, followed by the appropriate private mailbox number. Use of any other identifier is prohibited. Either a three line or four line address format can be used with a CMRA address and the PMB or # identifier, with the exception noted below. Where the CMRA's physical address requires its own secondary address element, the PMB or # address must follow the specific format rules stated below. It is not permissible to combine the secondary address element of the mailing address for the CMRA and the CMRA customer's private box number.