New: swipe left or right in an open message to move to the previous or next message in the list, the way most mail apps work. The list's context (folder or unified inbox, sort, unread filter, and any active search) is kept, and older mail still loads as you reach the end.

Also: when the unread filter is on and everything is already read, the inbox now says so clearly instead of looking empty, with a one-tap way to show all mail.
