Use Ring Alarm Door Sensor For Mailbox?

Hello, I have a Ring Alarm setup that included three door sensors. I only have two exterior doors so this sensor has not been used. I recently thought it would be nice to make use this sensor for my mailbox so I can get an 'alert' when the mail has been delivered. The sensor does reach - with the use of the included range extender - and alerts occur when I test the two parts of the sensor inside the mailbox. It even works if I have the wider side (has the battery) mounted to the mailbox, but the smaller magnetic side is not mounted. As soon as I attach the magnetic side to the mailbox the sensor quits working. I know the two sensors are closer than on my doors as I put a camera inside the mailbox so I could be 100% how things lined up. This is a metal mailbox and am not sure if the magnetic side of the sensor being on the metal mailbox is the issue. I would love to use the sensor for this purpose so any thoughts, hints or solutions would be greatly appreciated. Thanks in advance, Chris