Let me start by saying that I'm a mainly a fan of GMail, Google's Web-based e-mail program. I use three different e-mail programs (for reasons I'll elaborate on some other time), but GMail is certainly one of them.
One of the cardinal rules of interface design is Don't move buttons around! Buttons should be in consistent locations from screen to screen. This is largely because of what's affectionately known as muscle memory ... the ability our bodies have to remember physically how to perform an action. It also has to do with consistency in general, which makes interfaces more predictable and hence easier to use.
If, for some reason, you have to move buttons around, the worst thing you could do would be to reverse the meanings of the buttons. For example, if you have a "Save" button on the left and a "Delete" button on the right in one screen, you don't put the "Delete" button on the left and the "Save" button on the right in another screen. You can imagine the consequences.
With this in mind, I now present, without further comment, two portions of screen shots from GMail. The first is the button row at the top of the "Inbox" view:
The second is the button row at the top of the "Spam" view: