Hello Louis-Alain
You are correct; there is no built in functionality at present that limits the number of emails a recipient can be sent in a given time period. You could look into your plug-in idea to handle this, or you could set up a workflow that could temporarily unsubscribe a recipient for a given length of time after they have received a certain number of emails which you could track with a custom counter field that you add to the lead/contact record (although this option would not be very flexible).