In order to delete a message you sent to someone you have to do it from 'tracking' and obviously before the person has actually opened it (which 'tracking' should let you know). However, the 'tracking' feature has been inconsistent in terms of lag. I know it's gotten slower and slower for me to a point where I can't even use it any longer. Seems to not happen to everyone nor is it all the time. But that's the deal and hopefully in the future the lag/inability to work will be fixed with that.