Hmm. I realized that a common task for me (and probably for others) is to reply to a message and archive or delete it. So it'd be nice to continue to support doing that with a single 'submit'. A drop-down over the 'select' check boxes with a list of folders and 'delete' would work well for that, I think (while also having a "Move" button for clarity).

Though, 'submit' can trigger both sending a reply and moving/deleting... Should the 'delete' / 'move' buttons refrain from sending a reply? Probably not.