What occurs when mail is marked as "Unable to Forward"?

When mail is marked as "Unable to Forward," it indicates that the postal service has attempted to redirect the mail to a new address, as per the recipient's forwarding request. However, if they are unable to do so—perhaps because the forwarding order has expired or the new address is invalid or unknown—the mail will be returned to the sender. This process ensures that the sender is informed of the issue and can take appropriate action, such as verifying the address or updating their mailing list.

The other options involve actions that do not align with the postal service's policy regarding undeliverable mail. Holding the mail for pickup implies the recipient could still claim it, which is not the case when forwarding is no longer viable. Delivering the mail to a neighbor is not a procedure that occurs under such circumstances, as mail can only be redirected to the intended recipient or returned. Discarding mail would violate postal regulations as it disregards the established process of returning undeliverable items. Thus, returning the mail to the sender is the standard procedure when mail cannot be forwarded.