This is a mailbox flag that lets you know if the mailman has come or not. It also let you know if some other family member has already taken in the mail. My mailbox is a Gibraltar model ES15.

You may not want to use this if you are concerned with people stealing your mail since this will also alert them there is something in your mailbox.

UPDATE: I updated the design to have a larger pin/bolt holding on the flag. This also helps hold the whole thing to the mailbox better. Also, the threads are able to be larger and more durable.