Why are mail boxes "approved" by the postmaster general?
On my walk back from work yesterday, I was walking past a row of free standing mailboxes, all with the phrase “Approved by the postmaster general” on them. It got me thinking, are there any mail boxes that are not approved? And if so, does the postal service reserve the right to not deliver to those boxes? Why does a box have to be “approved”?
This question is in the General Section. Responses must be helpful and on-topic.