What classifies a mailpiece as anonymous mail?

A mailpiece is classified as anonymous mail primarily because it lacks legitimate sender identification and is often designed to conceal the sender's identity. The specific characteristic that defines anonymous mail is the presence of a fake return address. This is significant because anonymous mail is intended to be sent without any possibility for the recipient or postal authorities to trace it back to the original sender.

While the other characteristics might imply a lack of sender information, they do not specifically denote anonymity. For instance, the thickness of the mailpiece or the weight alone does not indicate whether it was sent anonymously; those options could potentially still provide valid sender information.

The key feature of using a fake return address underscores the intent behind anonymous mail, as it actively misleads the receiver regarding the origin of the correspondence. Thus, the defining characteristic highlights the core concept of anonymity in mail handling.