You can use the Gradebook to send messages to your students. Message subjects are filtered based on specific assignment categories:

• Haven't submitted yet—students who haven't submitted the assignment. This category does not include students who have been manually awarded a grade, even if they did not submit the assignment. Once a grade has been awarded for an assignment (either automatically or manually), Canvas no longer verifies actual submissions.
• Haven't been graded—students whose assignments have not yet been graded (submitted or unsubmitted).
• Scored less than [point value]—students who earned a grade on their assignment less than X number of points.
• Scored more than [point value]—students who earned a grade on their assignment more than X number of points.  

Although one message most likely will be sent to multiple students at the same time, each student will receive an individual message.