I have been in touch with Mark about this and I feel (and I thinkOh.
In that case I would suggest that the Post button be careful, and if the user is already on the Post Message page it should put up a "You haven't sent this message" banner, or something like that, without disturbing any content the user may have already put into the Subject or message body boxes.
That would clue the user that the buttons on the left are for changing pages, not taking actions. It would also give the user an opportunity to look again and connect the word "sent" in the message with the word "Send" in the button below the message box.
Also, by the same logic, the Home and other buttons in the left column should give the user a warning or "are you sure" pop-up or something if the user tries to switch away from the Post Message page with any content typed into that page. Right now they also cause the user's text to be discarded, which could be a big disappointment.