In the new version 1.1.0, mailwatch doesn't change its status when you click on the icon. After reading your email, you have to manually tell mailwatch to update so that it goes back to no messages.
The previous mailwatch 1.0.x would change its icon once you clicked it.
It would be nice if there was an option to toggle between the two behaviors.
Er, it's never done that. Left-click runs the command in the prefs dialog, middle-click forces an update.