The Inbox is a messaging tool used to communicate with a course, a group, an individual student, or a group of students. You can use the Inbox to communicate with others in your course.
Notes:
- Users display in the Inbox once they have an active enrollment in the course, and users cannot join a course unless it is published.
- Once a course has concluded and its term date has passed, you can no longer message students in that course.
From Canvas Basics Guide > What is the Inbox?