What indicates that mail is accountable?

The indication that mail is accountable primarily hinges on the use of a specific tracking number. This number serves as a unique identifier for the piece of mail, allowing both the sender and the recipient to track its movement through the postal system. Accountable mail typically requires additional handling and security measures, which are documented through this tracking system.

While other factors, like delivery to a mailbox or a signed PS Form 3849, might relate to mail delivery processes, they do not inherently signify accountability. A continuous chain of receipts may indicate that a transaction is being tracked, but it does not provide the same level of specificity as a unique tracking number does. Thus, the correct response highlights the formal mechanism by which accountability in mail processing is typically established and monitored.