I know we all LOVE  LOVE  LOVE  the "Message Students Who" feature from the Gradebook.   

The one filter that doesn't seem right to me is the "Haven't been graded" option, because it includes the "Students who haven't Submitted."

If this filter selects students who submitted but for whom a teacher hasn't had an opportunity to grade yet for whatever reason, they can message those students and let them know why and when they can expect to see a grade and feedback.