When we view an email with lots of recipients in the 'message preview' pane, or the dedicated 'read message' window, the list of addresses in the 'To:' field is contracted. Currently, a 'left-pointing double arrow' is displayed when the list is contracted, and a 'down-pointing double arrow' is displayed when the list is expanded.

I feel that the display should actually be inverted, i.e. the symbol diplayed should indicate what the button will do, rather than indicating the current state. This makes sense since this is a clickable element that performs an action (similar logic is used for play/pause buttons in music players, for instance).