Post Office rules, not always followed but they're supposed to know the last names of everyone receiving mail at an address or something like that. I put a large sticker on the inside of my mailbox, where the postal carrier can see it easily, with 3 different last names on it (mine plus 2 others). Have never had any trouble with getting all the mail delivered.
EDIT: Or, I could have said "What Funky Ghost said".
