The main issue with posting to only one person is that the person doesn't know it is actually private, at least it didn't have any marking the last time I tried

The main issue with posting to only one person is that the person doesn't know it is actually private, at least it didn't have any marking the last time I tried. The feature would be perfect if there was a message saying "this post have been shared with you exclusively, your replies will only be seen by the original poster".