Flag that you put on your mailbox door.  When the door opens the flag falls forward to let you know that it has been opened.  Super simple, but super handy.

I just used a magnet from Harbor Freight 22x28x10mm.

I also painted the exposed side of the “flag” to match my mailbox.  That way it is subdued when closed and really shows up when extended.

New and Improved Version Available

https://www.printables.com/model/455507-magnetic-mail-arrival-flag