I have worked for the U.S. Postal service, as a mail carrier, for the last 13 years. I have noticed these dots too. I assure you that I have never placed any dots on any mailboxes or posts, and I have never heard this subject discussed by any carrier or management. I am pretty sure that the dots are placed there by your newspaper delivery person to indicate which papers you subscribe to.

To test this, why don't some of you who get newspaper delivery discontinue one or all of your newspapers and see if the dots get taken off your mailbox or post.

I think I will bring this issue up at work. According to postal policy, no one is supposed to be using the MAILBOX except for the U.S. Postal Service. The MAILBOX is supposed to be under postal jurisdiction and no one is supposed to place ads inside or on the outside of the MAILBOX, including the flag on the mailbox. Many people, and businesses, erroneously believe they can attach ads to the mailbox FLAG or on the outside of the mailbox. The POST however is open territory. Many mailbox POSTS manufacturers now include a space under the mailbox for the placement of newspapers or advertisement flyers.