What qualifies as Anonymous Mail?

Anonymous Mail is specifically characterized by its lack of identifying information about the sender. The correct option defines a specific scenario of mail that does not include identifying sender information, particularly focusing on the weight and presentation of the mail.

When a piece of mail exceeds 13 ounces and is not presented at retail, it typically requires clearer identification and handling from the postal service due to its characteristics. This helps in ensuring proper tracking and delivery. Without a retail presentation, the sender's details are not readily available, which can lead to the classification of the mail as anonymous in nature.

The other options describe different aspects of mail handling and conditions but do not align with the formal definition of Anonymous Mail as effectively. For instance, mail without a return address could still have sender identification through means other than the return address, such as the recipient's knowledge of who sent it. Similarly, mail sent from a P.O. Box can still include identifying information, and mail that lacks a sender's name may not always meet the criteria if there are other identifying factors present.