Abstract: A receiver unit (2) for a radio signal-based mail delivery system comprising a transmitter unit (1) for being mounted on a mailbox or mail slot flap (4) and such receiver unit (2) for being located at a distance from the transmitter unit (1). The reciver unit (2) is provided with a receiver for receiving a radio signal and with means for indicating that such radio signal has been received. Besides, the receiver unit (2) is provided with a clock and is configured for registering the hour when the radio signal was received; and the means for indicating when a radio signal was received comprise a visual and/or audible message about the hour when the radio signal was received also including a mail delivery system comprising such receiver unit.