So, he opens the box and picks up the letter. He glances at it briefly to make sure it is stamped, and then puts it in his bag with his other letters. When he gets back to his office, the letter goes in a big bin with all of the other out-going mail. He thinks nothing of it, and finishes his day.
A few days later, he delivers the exact same letter to the same house that he picked it up from. He glances at the mailing address and observes that it is indeed the same as the return address. Anyone can make a mistake, so the mailman puts the letter in the mailbox so that the customer can readdress it for its proper recipient. He thinks nothing of it and finishes his day.
The next day, the mailman sees the same mailbox with the flag up. He opens the box and again sees the same letter, nothing changed, but with a new stamp on it. The mailman is perplexed, and thinking to save the customer both time and money, decides to ring the doorbell and inquire about the letter.