Do you check and empty your mailbox regularly? If you let your box fill up or if it hasn't been picked up in 10 days, it's possible the regular marked it as vacant. All mail would have been pulled and held at the office for 10 days before being returned to the sender. You need to go down to the office that serves you and let them know that your address isn't vacant and what good names are for your address. It sounds like you got marked vacant for not keeping the box empty.