When a message is deleted (while you are still in the message), the next message in the view is opened. The user may not want to read or act on that message, and in some cases, may mark the message as unread in order to remember to return to it later. It would be preferable to be returned to the inbox view, so that the user has control over the next message that is read. This change would also be consistent with how other actions taken while reading mail are handled, i.e.
So in other words, when acting on an open message (forward, reply, or delete) when that action is completed, return to the inbox view.