There is a feature in Outlook that can be activated to 'Work Offline'.
When activated, all emails sent will be stored in 'Outbox' and Outlook won't connect to the server to retrieve or send any emails.
The button for this can be found on the Send/Receive ribbon, as shown below
The status bar at the bottom-right of Outlook will then indicate that you are offline.
To switch back to Online mode, simply click the 'Work Offline' button once, and the status will be reflected in the bottom-right of Outlook
