Postal items that cannot be delivered to the receiver because of problems such an erroneous address, insufficient postage, or the recipient's unavailability are referred to as undeliverable mail. If a return address is given, these items are usually mailed back to the sender; if not, they are usually forwarded to a dead letter office. Address errors, out-of-date forwarding information, or recipient denial are common causes. Usually, updating or correcting postal information is necessary to resolve such problems.