am facing the same issue and I am using Moodle 3.5+
My desired scenario: No body but teachers can message to any and all on the participants list inside of a course.
Permissions (for moodle/site:sendmessage'):
Authenticated user: Not Set'
Student: Not Set
Teacher: Allowed
Result: Teachers are unable to send messages because of error claiming that permission for above flag is not set.
Per your message, if Teacher is to be able to send messages then so can any authenticated user including students which vitiates achieving my goal. Looks like it is quite a common goal.
Is my understanding correct?
Thanks
The main issue with the sendmessage permission is that it is all or nothing. If you deactivate it, then student's can't send teacher's messages as well. So to prevent them from messaging students, you have to ALSO prevent them from messaging teachers.
What most people want is to enable teacher-to-student messaging only. Unfortunately, moodle messaging doesn't support this, and you have to hack moodle code to make it work.
For 3.4 and 3.5, if you are willing to add a few lines of code, it makes it happen: