The landmark became a meeting place for UFO buffs. Someone spread the idea the mailbox contained secret documents going in and out of Area 51 (because, yeah, that's the way the Air Force handles its secrets) and they started poking through the rancher's mail. Some even left letters for the government. And for aliens.
The rancher got fed up and replaced the Black Mailbox with one that locked. It was white, but UFO fans still called it the Black Mailbox. It's rumored the original Black Mailbox was auctioned off to a lucky UFOlogist.
Now the white Black Mailbox is gone. It used to sit atop this post.