Actually I'm pretty sure this isn't true. I remember doing some research on P.O. boxes a while back when working on the old SR wiki, and I went to a UPS store to ask about their mailbox program. One of the main things they made very clear was that only mail addressed to the mailbox owner's name would be accepted, and any mail that wasn't addressed to said person's name would be sent back as undeliverable. I am pretty sure this is a post 9/11 U.S. nation-wide law that applies to all private businesses that offer mailbox services.