For years  this was the best Android mail app available. It's unbelievable how the current developers have ruined such a fantastic email utility. Many of the old behaviors have been modified to work in unintuitive ways  demonstrating a lack of basic user ergonomics. The Unified Inbox in particular is a catastrophe. Now  unread messages show before read messages regardless of date or time. Read messages do not show at all (they completely disappear from Unified Inbox) UNLESS YOU READ THEM WITH ANOTHER CLIENT (such as your desktop PC). If you read a message with another client  it will appear below the list of unread messages  completely out of time/date order. The Unified Inbox worked perfectly in older versions  but whoever has been modifying the code for this section of K-9 is essentially incompetent. There should be settings to configure the Unified Inbox behavior back to the old method if a user prefers that. It is after all the reason most of us switched to K9 in the first place. But the current behavior of showing messages read on another client in one way  and messages read in K-9 in another way  is just extremely poor development  and it needs to be fixed.
February 14 2017