The Set Mail Status action works on a selected message in Apple Mail.
Note that the currently selected mail message is not necessarily the front window, it may be a message selected in the viewer behind the front window, especially if the front window is a reply or new message.
Select which status to set using the Set Mail pop-up:
Set the status using the to pop-up:
Note: if you select value then off and on will no longer be available.
If you choose value then you must enter a number or choose one from the stepper control.
Note that in Apple Mail flags may be: