Send system message to user i would like to send system message to user such as site announcement, notification and etc. 

i know the private message module, but it could not differentiate the message is send by the real system/admin or a normal user.  

How should i do that??
Try the notifications framework:


  This is a complete Subscriptions/Notifications Framework aiming at extendability and scalability. It allows any number of plug-ins defining new event types or subscription types or a different user interface.


Its a full featured module with a number of companion modules for sending messages via different channels depending on different events but what you're probably looking for is the 'Web messaging' submodule:


  Sending Method: Show messages on user's page