Iirc, private message goes from outbox to Sent Messages once read. For certain topics seen by someone you will need to do custom coding as you make sure that you don't kill the purpose of hidden users. Marking messages as seen in-game is a different thing. Message window pops up right away, but that doesn't mean user actually saw the message given that they might be away or something else. In order to apply that in-game first thing you need to do is to code the system that messages does not pop wide open instantly due to above said reason. Once that is done a returning trigger can be done if the user you are communicating with actually opened the window that was previously minimized. Personally, I don't see any use of that. Just more information going around the server for no reason.