How do I determine who site participants can send a message to?
This feature gives the option to regulate each role's message composing permissions.
Go to Messages.
Select the Messages too from the Tool Menu in your site.
Click Permissions.

Permissions is located below the Course Site title.
Check the corresponding boxes for desired permissions.

Permissions are assigned on a role-by-role basis. For instance, to prevent a student from sending a message to "All Participants", un-check the Student Permission box located next to the option, "Allow the 'All Participants' options to the 'To' field."
Note: These options will be visible when clicking the "To" text box in Compose a Message, within the Message tool.
Use drop-down menu for separate permissions based on groups.

This option allows the user to change permissions for different groups. This way, each group has unique Message tool options.
Note: By default, this option may only be available for admin users.
Click Save.

Click Save or Cancel to quit.