This is a small module called Client Notifications that allows you to send messages to individual clients or all your Clients at once. With this module, you can send the single message to individual clients or all clients. Once the member login they will see all the messengers received on the front page.
Example: You choose a client and a send message. No one else will see that message but only the intended client. Then if at a later date you decide to send more messages you have the option...