checkmark mail messages

At this moment, when you send an user a Owl/PM/BM/Mail, you have no idea if the user you send it too have read it. I would like to see that there will come a function that you can see that an user has read/opened it. This way the user can't say that they haven't read it, like they can do now. 
Just a simple checkmark would be sufficient, and would stop the endless wondering that the sender could have at this moment.