To let others know you are not available (from Outlook Express)
On the Tools menu in Outlook Express, point to My Online Status, and then click the item that best describes your status.
Your status is displayed when the mouse pointer hovers over your name (not as in the messenger service program, where it is displayed in parentheses after your name). Also, the icon beside your name changes.
Note
You can still receive messages, but if you click Busy or On the Phone, an incoming message will not flash in the taskbar or make the usual sound.