I get push notifications from an Aeon contact sensor when my mailbox is opened. However, sometimes I get false signals for multiple reasons. Thus when the door is opened, I sometimes want to check it remotely to see if there is any mail and maybe the contents. And a camera may help catch a thief.
So I added a Blink camera to the door:
Here is how it looks from inside the mailbox:
You can see the Aeon sensor on the opposite door.
Of course the Blink is not rated for outdoor use.
EDIT: To get a more flush mount, drill a hole in the knockout and screw it to the door. To stabilize it, super glue the knockout to the cover.
This 1/2 inch reduction brings about 1 1/2 inch more of the mailbox contents into view.