Well, how LetterBomb works is that it reuses the Wii Message Board for homebrew reasons. It is indeed defunct (whatever that means; outdated, maybe?), and since the Wii can read backed-up messages from the SD card, some good developers have made a fake Wii Message (or Wii email service message, whatever you like to call it) to trick the Wii into thinking that it (the Wii) made (and backed-up) the message. Since the Wii does not “know” its mail service is no longer working, that’s where LetterBomb comes into play. The Wii can back up messages when its own NAND (you know, internal storage) is full and needs more room. The LetterBomb website (please.hackmii.com) needs your Wii’s MAC address to sign the correct file, as the Wii only likes messages sent to its MAC address. Sorry if I am making no sense.
EDIT: You need the right date because the Wii basically “thinks” it backed up the message at that time, LetterBomb has yesterday’s date “stamped” on it.
EDIT 2: You set the date BEFORE you try to run LetterBomb; on the inside LetterBomb is just an ordinary, backed-up Wii message with a lot of code inside it.