Letters to students in a course are sent from the student list through the office correspondence - to one or several selected ones at once.
To send a newsletter to your students, open the Courses section, select a course, and then the "Students" tab. To message one student, open the "⋯" menu next to the row and select "Message." To message multiple students, select their checkboxes and select "Message" or "Remind about course" (the course link is already included in the message). The messaging modal will open.
These are letters from your mailbox account, not a mass newsletter designer.