I do not know anything about the Outbox startup other than what is presented in the linked article, but I do know that this is not an accurate representation of the approach of the Post Office to electronic mail. They considered a system almost exactly like this in the late 1970s. It was called E-Com, and it allowed users to send letters electronically from office to office. The letters were then printed out and delivered.
The Post Office might have its flaws, but from the 1792 Post Office Act to the present, it has actually been an important contributor to the information infrastructure of the United States. This article reads like a press release from the start-up in question.