I just sent myself 3 new emails and replied to one :-) and I think what you might be seeing is it says "inbox" in the right corner? That means you are in the outbox, but you can push that link to go to the inbox if you want. It stayed in the box I was in while I was working on that box. When I got to the "end" of Next/Previous, then it went to that index for the box I was in. But when you're in the inbox the upper right says "outbox" and when you're in the outbox it says "inbox" because that's the link between the two to switch from one to the other from whichever one you're in.

It's very easy to get confused because most people reply to posts by the same name as those they received so they get several going back and forth in inbox and outbox with the same names.