How do I determine who site participants can send a message to?
This feature gives site owners the option to regulate each role's message composing permissions.
Select Messages.

In the target Sakai site, select the Messages tool from the Tool Menu.
Select Permissions.

Select Permissions from the actions menu.
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 send to "All Participants" option.
Note: These options will be visible when clicking the "To" text box in Compose a Message, within the Message tool.
Select Save.

Select Save to save your changes.