The Messages tool allows you to reply to a message sent to you by another member of the course.
Select the Messages tool from the Tool Menu in your site.
Select the Received folder to view a list of your received messages.
Select Reply to reply to the author of the original message. (Or, select Reply to all to reply to all parties included on the original message.)
Compose the message and select Send. For instructions on composing a message, view the article How do I send a message?