Yes this is possible.
The Smart Folder shows messages which - over a selectable time period - :
1) were sent (this is the date/time in the header);
2) were read, including read in other apps (for IMAP).
3) or are unread, no matter what their date/time is.
You can also include starred messages or even Sent messages in this list, please see app settings under SmartBox or the navigation drawer (side panel) in the SmartBox’s message list view.
To set a folder (any folder, not just Inbox) whether it’s included or not in the SmartBox / Smart Folder or to asdign a folder a different color, please do this:
1) long-press the account in the main window
2) choose “Options and folders”, then “Folders”
3) scroll to whichever folder you wish to change
4) long-press the folder or tap the settings icon (wheel) to the right of the folder’s name
5) Enable /disable the option "Show in SmartBox"
Per-folder color labels are enabled by default, but can be disabled in settings.
