What is the purpose of the Mailbox in the Bastion?

At the end of my second playthrough of Bastion, I was roaming around one last time around the Bastion area and noticed, to my surprise, a mailbox at the bottom right corner of the central hub. I hadn't seen this before, but apparently it had been there most of the game. I can interact with it, but I just get a witty comment or two from the narrator.

While I can't imagine that the Bastion gets much mail, I feel as though there might have been some sort of purpose for this mailbox other than atmosphere. Am I wrong, and is the only reason for the mailbox to add atmosphere? Or is there some sort of reward or backstory revealed for checking in on the mailbox often or at the right times?

Solution 1:

No one has seen the Outhouse or Mailbox serve any useful purpose. They basically just make the "I can't open this" noise when you use them.

There are rumors on the Internets (actually, speculation is the better term but I can't resist the urge to use the meme) that the Mailbox could be how you access future downloadable content:

• GiantBomb Forums
• Steam Forums
• Gamespot Forums

Solution 2:

At one point in the game, you deliver a letter from Zulf to Zia. I speculate in a development version of the game, the letter arrived in the mailbox.

There's a dream level accessible from Ruck's history books at the bastion, the stranger's dream. If you survive to the letter X (there are 26 stages), he mentions the mailbox.

It's plain enough what the outhouse is for. I hope you've been using it, or they'll be another calamity.