I just read an instructable about a Mailbox phone alert which was very good, but required electronic skills I do not have. I made this one without any electric power and it works flawlessly. Open the door and a metal flag drops down to let me know the mail carrier has delivered the mail. I can see the mailbox from my front door so when the flag drops down it is clearly visible. No electrical parts, just powered by gravity. I welded the parts onto the mailbox, but one could be made by bolting everything on.
A pin on the flag arm goes into a hole in the door. I welded on a washer for the flag arm pin to go into when the door is closed. I could have bolted on a metal tab with a hole in it. The flag arm is heavier on the end with the flag, so it drops down when the door opens.
The wire on the bottom is to dampen the sound when the arm falls down so it does not startle the mailman. It is easily reset with one hand when I am in my car.