Abstract: A mailbox including an indicator indicating the presence of mail, if any, and including a detector having a transmitting and receiving ultrasonic transducer. The transducer transmits ultrasonic waves into the mailbox and based on the received ultrasonic waves indicates alterations in the reflected energy. In this manner mail in the mailbox is detected even if the mail does not take up much room. The detector detects alterations in the frequency as a consequence of movements in the mailbox or detects alterations in the pattern of reflection as mail in the mailbox is disturbing the propagation paths of ultrasound in the mailbox.