In Outlook, you have the flexibility to make Outlook work online or offline with your mail server.
How do you know when you're working offline or online in Outlook? There are visual cues, starting with the status bar at the bottom of your Outlook window. If either of the following status indicators are shown, it means Outlook isn’t connected to your mail server.
or 
You can’t send or receive email until you reconnect.
Working offline usually is a choice. Returning to an online work mode is a simple process; In Outlook, on the Send/Receive tab, choose Work Offline to reconnect:
